WIP: Tmuxinator 0.6.0

This pull request will contain the development for version 0.6.0 of tmuxinator. My goal is to clean up the code, increase test coverage and fix outstanding bugs for this version. Additional features can then be attempted for 0.7.0.

- [ ] Write specs for each of the classes, attempting to get coverage >= 80%
- [ ] Clean up command completion code, taking a similar approach to how [rbenv does it](https://github.com/sstephenson/rbenv/tree/master/completions)
- [ ] Fix outstanding [bugs](https://github.com/aziz/tmuxinator/issues?direction=desc&labels=bug&page=1&sort=created&state=open)