Remove undreline

Underline fontStyle is a legacy option from TextMate. Sublime does not
support it, so it's time to abandon it.
Implement @FicheFoll's suggestion for streamlining the fontStyle mixin
Add support for plugins: SublimeLinter & GitGutter

- SublimeLinter has not been tested
- GitGutter was already partly supported, blended in with the
Critic Markup selectors.
Consolidate GitGutter styles with Critic Markup