Add keybinding to trigger linting manually

Default keybinding copied from the Sublime Text linter.

Fixes #146.

Test Plan:

  * Opened Atom, turned off lint on focus/modify/save
  * Reloaded the window and edited code so that there was a lint error
  * Pressed `cmd-ctrl-l` and saw a lint error appear
  * Fixed the lint error and pressed the bind again
  * Lint error now gone
Merge pull request #149 from dmnd/add-key-binding

Add keybinding to trigger linting manually