Add Folder.createSubfolderIfNeeded()

Just like its FileSystem equivalent, this method either creates or returns
an existing subfolder for a given name.

Sources/Files.swift

@@ -520,6 +520,21 @@ public final class Folder: FileSystem.Item, FileSystemIterable {
             throw Error.creatingFolderFailed
         }
     }
+
+    /**
+     *  Either return an existing subfolder, or create a new one, for a given name
+     *
+     *  - parameter folderName: The name of the folder to either get or create
+     *
+     *  - throws: `Folder.Error.creatingFolderFailed`
+     */
+    public func createSubfolderIfNeeded(withName folderName: String) throws -> Folder {
+        if let existingFolder = try? subfolder(named: folderName) {
+            return existingFolder
+        }
+
+        return try createSubfolder(named: folderName)
+    }
 
     /**
      *  Create a sequence containing the files that are contained within this folder

Tests/FilesTests/FilesTests.swift

@@ -383,6 +383,17 @@ class FilesTests: XCTestCase {
             XCTAssertEqual(subfolderA.files.first, subfolderB.files.first)
         }
     }
+
+    func testCreateSubfolderIfNeeded() {
+        performTest {
+            let subfolderA = try folder.createSubfolderIfNeeded(withName: "folder")
+            try subfolderA.createFile(named: "file")
+            let subfolderB = try folder.createSubfolderIfNeeded(withName: "folder")
+            XCTAssertEqual(subfolderA, subfolderB)
+            XCTAssertEqual(subfolderA.files.count, subfolderB.files.count)
+            XCTAssertEqual(subfolderA.files.first, subfolderB.files.first)
+        }
+    }
 
     func testUsingCustomFileManager() {
         class FileManagerMock: FileManager {