Allow "jj" to exit insert mode in Atom (working as of 1.0.2)

gistfile1.coffee

# Slightly modified version created by rdlugosz at https://github.com/atom/vim-mode/issues/334#issuecomment-85603175
#keymap.cson
'atom-text-editor.vim-mode.insert-mode':
  'j': 'exit-insert-mode-if-preceded-by-j'

#init.coffee
atom.commands.add 'atom-text-editor', 'exit-insert-mode-if-preceded-by-j': (e) ->
  editor = @getModel()
  pos = editor.getCursorBufferPosition()
  range = [pos.traverse([0,-1]), pos]
  lastChar = editor.getTextInBufferRange(range)
  if lastChar != "j"
    e.abortKeyBinding()
  else
    editor.backspace()
    atom.commands.dispatch(e.currentTarget, 'vim-mode:activate-command-mode')