Add new host option to helpers

This provides a workable solution to #53 until a larger scale architecture solution can be made.

It adds a `host` option to the `attachment_url` helper. It is optional and if not used the behavior is the original behavior.

I also added some basic testing around the helper.

Example usage:
`attachment_url(user, :avatar, 'fill', 100, 100, filename: 'avatar', format: 'jpg', host: '//123abc.somecdn.com')`

I find this style of putting the expected result in a `let` a bit strange.