gtfo 2.0

New features:
- `g:gtfo#terminals` option to specify the terminal if you don't like
  the default(s).
- move logic to autoload/ (micro-optimization)

Breaking changes:
- remove `g:gtfo_cygwin_bash` setting. Use this instead:
- remove legacy `goo` mapping. See README.md to change the terminal.
- rename `gtfo#openfileman()` => `gtfo#open#file()`
- rename `gtfo#openterm()` => `gtfo#open#term()`