New Add/Remove prefix option (#69)

* New Add/Remove prefix option

Some libraries like [the gorm ORM](https://github.com/go-gorm/gorm) or
the [gaum SQL query builder](https://github.com/shiftleftsecurity/gaum)
use a prefix in the tag name (ie `gorm:"column:struct_field"`,
gaum:"field_name=struct_field") to help ther Scanner/Valuer interact
with the sql library in go.
This PR adds the ability to add and remove said prefixes.
I used these two libraries as samples in the tests files to help
understand the rationale behind said feature to people reading without
context, hopefully this doesn't break any project convention.

* Add Readme section for prefix

* Replaced prefix with format

* Update main.go

Accepting fatih's suggestion on flag doc

Co-authored-by: Fatih Arslan <fatih@github.com>

* Update README.md

Accepting fatih's suggestion on `format` flag

Co-authored-by: Fatih Arslan <fatih@github.com>

* Update README.md

Accepting fatih's suggestion on readme secion title

Co-authored-by: Fatih Arslan <fatih@github.com>

* Rename formatting var to valueFormat

* Rename $value to $field for clarity

* Update readme too

Co-authored-by: Fatih Arslan <fatih@github.com>

README.md

@@ -149,6 +149,27 @@ type Server struct {
 }
 ```
 
+### Formatting tag values
+
+By default a struct tag's value is transformed from a struct's field and used directly. As an example for the field `Server string`, we generate a tag in the form: `json:"server"` (assuming `-add-tags=json` is used).
+
+However, some third party libraries use tags in a different way and might require to them to have a particular formatting, such as is the case of prefixing them (`field_name=<your_value>`). The `--format` flag allows you to specify a custom format for the tag value to be applied.
+
+```
+$ gomodifytags -file demo.go -struct Server -add-tags gaum -format "field_name=$field" 
+```
+
+```go
+package main
+
+type Server struct {
+	Name        string `gaum:"field_name=name"`
+	Port        int    `gaum:"field_name=port"`
+	EnableLogs  bool   `gaum:"field_name=enableLogs"`
+	BaseDomain  string `gaum:"field_name=baseDomain"`
+}
+```
+
 ### Transformations
 
 We currently support the following transformations:

main.go

@@ -66,6 +66,7 @@ type config struct {
 
 	transform   string
 	sort        bool
+	valueFormat string
 	clear       bool
 	clearOption bool
 }
@@ -113,6 +114,10 @@ func realMain() error {
 		flagSort = flag.Bool("sort", false,
 			"Sort sorts the tags in increasing order according to the key name")
 
+		// formatting
+		flagFormatting = flag.String("format", "",
+			"Format the given tag's value. i.e: \"column:$field\", \"field_name=$field\"")
+
 		// option flags
 		flagRemoveOptions = flag.String("remove-options", "",
 			"Remove the comma separated list of options from the given keys, "+
@@ -146,6 +151,7 @@ func realMain() error {
 		clearOption:          *flagClearOptions,
 		transform:            *flagTransform,
 		sort:                 *flagSort,
+		valueFormat:          *flagFormatting,
 		override:             *flagOverride,
 		skipUnexportedFields: *flagSkipPrivateFields,
 	}
@@ -399,6 +405,10 @@ func (c *config) addTags(fieldName string, tags *structtag.Tags) (*structtag.Tag
 		unknown = true
 	}
 
+	if c.valueFormat != "" {
+		name = strings.ReplaceAll(c.valueFormat, "$field", name)
+	}
+
 	for _, key := range c.add {
 		splitted = strings.SplitN(key, ":", 2)
 		if len(splitted) >= 2 {

main_test.go

@@ -41,6 +41,26 @@ func TestRewrite(t *testing.T) {
 				transform:  "snakecase",
 			},
 		},
+		{
+			file: "struct_format",
+			cfg: &config{
+				add:         []string{"gaum"},
+				output:      "source",
+				structName:  "foo",
+				transform:   "snakecase",
+				valueFormat: "field_name=$field",
+			},
+		},
+		{
+			file: "struct_format_existing",
+			cfg: &config{
+				add:         []string{"gaum"},
+				output:      "source",
+				structName:  "foo",
+				transform:   "snakecase",
+				valueFormat: "field_name=$field",
+			},
+		},
 		{
 			file: "struct_remove",
 			cfg: &config{

test-fixtures/struct_format.golden

@@ -0,0 +1,6 @@
+package foo
+
+type foo struct {
+	bar string `gaum:"field_name=bar"`
+	t   bool   `gaum:"field_name=t"`
+}

test-fixtures/struct_format.input

@@ -0,0 +1,6 @@
+package foo
+
+type foo struct {
+	bar string
+	t   bool
+}

test-fixtures/struct_format_existing.golden

@@ -0,0 +1,6 @@
+package foo
+
+type foo struct {
+	bar       string    `gaum:"field_name=bar"`
+	timestamp time.Time `gaum:"@timestamp"`
+}

test-fixtures/struct_format_existing.input

@@ -0,0 +1,6 @@
+package foo
+
+type foo struct {
+	bar       string
+	timestamp time.Time `gaum:"@timestamp"`
+}