Add public XDG_RUNTIME_DIR method

[chrisduerr]

This changes the visibility of the existing get_runtime_directory method to pub to make it possible for consumers to find that directory without requiring access to a specific file.

The documentation has been adjusted to match existing directory access methods.

Fixes #35.

[whitequark]

Thanks!

[chrisduerr]

Thanks for the super quick response. There have been a few changes since the last release 3 years ago (other than this most notably the state directory changes).

Would you be willing to pushing a new version in the near future?

[whitequark]

Released 2.3.0.