Upgrade right_prompt with branches, tags, + paths

Add new information to the right_prompt, including:

* Git branch information if the branch is not master

* Git tag information, or latest commit hash if there's no associated
  tag, with colored based on the repo's status (red for dirty, blue for
  clean)

* Git-relative pathing information, so when inside a git repo the path
  is displayed relative to the root of the project rather than the file
  system