Feat: option to remove (old) files

[nickschot]

Currently ember-cli-deploy-cloudfiles will only upload all new files. When fingerprinted files are uploaded or when some static assets are removed from the ember application, those files still remain in rackspace.

An option to automatically remove (some or all) old files in the rackspace container would be a nice feature to have. pkgcloud already has methods implemented to retrieve all current files in the container getFiles and a method to remove a file removeFile.

[jschniper]

@nickschot, I'd be happy to look at a pull request although I have some qualms with the plugin being responsible for deleting any files.