Add a "force" option to erb2haml rake task

[crawfoal]

I use the rake task in my application template, and I wish there was a way to force the task to delete the old erb files (and take sensible default actions on anything else that might come up) so that it doesn't require me to interact with the script.

[crawfoal]

I can work on a PR if this sounds like something you guys would be okay with

[syguer]

I understand such case. And I think it should be implement html2slim at first.
(It looks no discussion yet...)