Add script to echo public IP address
Update gitprompt for Git 1.8.5 compatibility

Git 1.8.5 changes the output of `git status`. [1] Long-term, I should
update this to script to use the `--porcelain` flag as suggested in the
release notes. In the meantime, this is a low-rent solution that seems
to get the job done.

[1]: https://github.com/gitster/git/blob/44484662d8/Documentation/RelNotes/1.8.5.txt#L123-L127