Get rid of GNU Grep dependency

[erikw]

Inspired by #466 it's a good idea to not have code using GNU Grep specific options like the convenient -P swithc, as systems like *BSDs and macOS comes with a simpler (bsd) grep not supporting these options.

Tasks

• Find all usages of grep and rewrite to use only standard tools.
  • Here are some obvious ones: https://github.com/search?q=repo%3Aerikw%2Ftmux-powerline%20%22grep%20-P%22&type=code