Another stab at factoring out InsertLineBreak command

Hi again - I couldn't figure out how to reopen the other pull request. Hopefully this is OK to just submit again. I got the factored InsertLineBreak command working sufficiently to pass all lint tests and saucelabs tests. I ended up taking an approach through which EnterKey.js passes the current event along to the command so that the old behavior is exactly mimicked when there is in fact a JavaScript enter key command being handled. In other cases where the client is calling the InsertLineBreak command without argument, it will just apply default behavior which I think is appropriate.