Add bash-completion script for golo cli.

This is my go at Issue #99. You can 'source golo-bash-completion', or drop it into your /etc/bash_completion.d/ (or /usr/local/etc/bash_completion.d/ on a Mac) and relaunch your terminal. 

I also wasn't sure where to put this file in the tree. This seemed like the best fit for the moment. I'll gladly change it if something else is preferred.