gettin started on working on backend

[Ragnyll]

Things that will have to change:

• Add crossterm_backend.rs and have it flip using a feature
• AppConfig:
  • This might not actually have to immediatley change. XDG open does lock this to using linux, but that's ok now. i just need the crossterm backend to work
• AppEvent: has some hard binds to termion

[Ragnyll]

This may be a little tricker than i thought. There are fundamental differences in the way that termion and crossterm execute, so just creating a drop in replacement may not work.

Right now I'm thinking the way to go about this is to create a Backend trait. then have both the termion backend and crossterm backend implement that.