Add an independent parameter to control whether columns are sortable.

[shellwalker]

If parameter format.sort is set to true , tables, columns, relations, constraints, and viewpoints will all be sorted by Name after Schema.Sort is called.

After execute the command tbls doc, it will generate an index file which named README.md in output directory doc/schema, the order of table, relations... keep the consistency, it looks good for me.

But for the columns' order in each table file(e.g., table-A.md), I still want to keep the default creation order instead of sorting them by name.

func (s *Schema) Sort() error {
...

  // if ( newSortFlag ) {
  sort.SliceStable(t.Columns, func(i, j int) bool {
			  return t.Columns[i].Name < t.Columns[j].Name
		  })
  // }
...
}

Thanks.