Add Folder.subfolder(atPath:)

This enables easier access to a subfolder located deep within a folder’s
tree, instead of having to retrieve all the intermediate folders.

Sources/Files.swift

@@ -480,6 +480,18 @@ public final class Folder: FileSystem.Item, FileSystemIterable {
     public func subfolder(named folderName: String) throws -> Folder {
         return try Folder(path: path + folderName, using: fileManager)
     }
+
+    /**
+     *  Return a folder at a given path that is contained in this folder's tree
+     *
+     *  - parameter folderPath: The subpath of the folder to return. Relative to
+     *                          this folder.
+     *
+     *  - throws: `Folder.PathError.invalid` if the folder couldn't be found
+     */
+    public func subfolder(atPath folderPath: String) throws -> Folder {
+        return try Folder(path: path + folderPath, using: fileManager)
+    }
 
     /**
      *  Create a file in this folder and return it

Tests/FilesTests/FilesTests.swift

@@ -135,6 +135,15 @@ class FilesTests: XCTestCase {
         }
     }
 
+    func testAccessingSubfolderByPath() {
+        performTest {
+            let subfolderA = try folder.createSubfolder(named: "A")
+            let subfolderB = try subfolderA.createSubfolder(named: "B")
+            let subfolderC = try subfolderB.createSubfolder(named: "C")
+            try XCTAssertEqual(folder.subfolder(atPath: "A/B/C"), subfolderC)
+        }
+    }
+
     func testEmptyingFolder() {
         performTest {
             try folder.createFile(named: "A")