Remove use of grep for base-index

I noticed that I had problems with tmuxinator due to the fact that I configure my GREP_OPTIONS to show me 3 lines above and below by default. After looking into the issue I realized that we could eliminate this problem by specifying the `-A 0 -B 0` flags so I wrote a few specs and done this.

After cleaning it up, I thought it may be a better idea to remove all reliance on the system grep and do all the work in ruby so I done this. This also prevents a second call out to `tmux show-options`.

Let me know what you think. I'm happy to make adjustments! Thanks for this project, I've really enjoyed it!