Allow configuration of additional allowed paths from which files may be embedded.

It would be helpful in some cases to be able to define additional paths from which MimeMail would allow files to be embedded. Currently the only allowed path is the defined public path. I would like to be able to embed from the theme path.

Alternatively, providing a hook for other modules/themes to determine if a path is allowable would also suffice to solve the problem.

I would be happy to provide a patch if this would be an acceptable feature.