Ability to refer file path directly

[ksoichiro]

Currently, gag can recognize only git URLs. While developing libraries, it is useful to apply changes of the libraries to the apps without commit. (Like CocoaPods)

With CocoaPod we can:

1. Use library on user-app: write pod xxx, :path => '/path/to/repo' to Podfile
2. Execute pod install on user-app
3. Modify library codes and test on user-app
4. If it's good, commit changed codes on library repo
5. Push changes to remote library repo
6. Change Podfile on user-app to refer remote library repo's new commit