Add a force option on build.

Adds a --force (or -f) option on building which will remove the target if it's a pre-existing directory. Hence making the "EEXIST" error avoidable if desired.

Fixes [broccoli-cli issue 4](https://github.com/broccolijs/broccoli-cli/issues/4)

This adds shelljs as a dependency, which is a bit of a strange library - so I understand if you're not keen on it :-) (I'd probably, personally, expect use fs-extra for this task - but don't mind, and shelljs seemed interesting :-) (and is already used by a broccoli dependency :-) )