Option to clear console between builds

[laurentpayot]

Elm compiler messages are quite verbose and the elm-live rebuilding message is only a couple of lines so I find it hard to distinguish what compiler messages are the current ones. A simple --clear option that would clear the console before builds would really make my day. What do you think?

[wking-io]

This sounds like a good first PR for anyone interested! Otherwise, I will look into adding this on my next round of updates.