Add FileSystem.createFolderIfNeeded()

This new method either returns an existing folder at a path, or creates
a new one if none exists.

Sources/Files.swift

@@ -323,6 +323,22 @@ public class FileSystem {
             throw Folder.Error.creatingFolderFailed
         }
     }
+
+    /**
+     *  Either return an existing folder, or create a new one, at a given path
+     *
+     *  - parameter path: The path for which a folder should either be returned or created at. If the folder
+     *                    is missing, any intermediate parent folders will also be created.
+     *
+     *  - throws: `Folder.Error.creatingFolderFailed`
+     */
+    public func createFolderIfNeeded(at path: String) throws -> Folder {
+        if let existingFolder = try? Folder(path: path, using: fileManager) {
+            return existingFolder
+        }
+
+        return try createFolder(at: path)
+    }
 }
 
 /**

Tests/FilesTests/FilesTests.swift

@@ -372,6 +372,17 @@ class FilesTests: XCTestCase {
             _ = try Folder(path: folderPath)
         }
     }
+
+    func testCreateFolderIfNeeded() {
+        performTest {
+            let subfolderA = try FileSystem().createFolderIfNeeded(at: folder.path + "one/two/three")
+            try subfolderA.createFile(named: "file")
+            let subfolderB = try FileSystem().createFolderIfNeeded(at: subfolderA.path)
+            XCTAssertEqual(subfolderA, subfolderB)
+            XCTAssertEqual(subfolderA.files.count, subfolderB.files.count)
+            XCTAssertEqual(subfolderA.files.first, subfolderB.files.first)
+        }
+    }
 
     func testUsingCustomFileManager() {
         class FileManagerMock: FileManager {