Add force option to "middleman init"

In one of my projects I would like to enable users to overwrite files without any question. I use `middleman` behind the scenes which does not provide a `--force`-flag when running `middleman init`. This PR adds this functionality. It makes use of the `Thor`-`force`-option and adds some tests. 

During development I also fixed one `rubocop`-offence and changed the order of `rubop` and `rake test`. `rubocop` fails very fast, but `rake test` runs a long time before failing. I suggest to change the order of those two. If you prefer leave it as it is, I can purge the commit from history. :-)