Launch the default editor to enter commit messages if `-m` is omitted

I suck at typing and editors do this cool thing called "spell check"
which, as you might have guessed, check your spelling. I'd like to
easily take advantage of that like I do with git. So now you can just
type `mm commit` and it will launch your editor, you can type your
commit message, save it, and then momentum will read your message and
send it along. Yay! :smile:
Better editor support through shellwords
Improved Gemspec with version bump
Merge branch 'editor-commit'