Added a fix for #408

I added a possible fix for #408; It's kind of a hack -- essentially, it checks if there's a keyEvent and patches `@value` with a newline character.

I probably messed up writing the spec; is there a way to insert a newline with `commandModeInputKeydown()`?

As this is literally my first contribution to anything Atom related, sorry if it's a woefully amateur attempt. Perhaps there's a better way to do this, but I have at least attempted to figure out the reason why this was broken before.