Towards pathlib.Path.mkdir compatibility

[smheidrich]

This PR aims to make the mkdir method of Plumbum's path classes (local & remote) more compatible with pathlib.Path.mkdir by implementing the arguments parents and exist_ok that are present in the latter. The mode argument has also been added to the argument list to match the signature of pathlib's method and preserve backwards-compatibility once it is implemented in the future, but I haven't implemented it yet, because pathlib does some umask-voodoo that I'd have to look into in more detail. I might add it later.

Should be backwards-compatible with earlier Plumbum mkdir behavior (which unfortunately required choosing defaults different from pathlib's, however) and of course comes with tests.

[coveralls]

Coverage decreased (-0.6%) to 82.492% when pulling e37791c on smheidrich:pathlib_mkdir_compatibility1 into efc2795 on tomerfiliba:master.

[henryiii]

I'd like pathlib compatibility (I'd love it if someday remotes were just subclassed pathlib paths), but we probably shouldn't make a backward incompatible default change until 1.7.0, for example.

[smheidrich]

Thanks for merging! 👍