Mail messages sent as plain text, doesn't allow html email

[instification]

Currently, the mail content rule just sends the message as plain text. This does not provide much opportunity for the author to format the email.

By creating a mime message before sending it would be possible to allow html email which could then be converted to a multipart message (and the text portion could be generated from portal_transforms).

To keep backwards compatibility, there could be a dropdown option on creating the content rule for the content type of the email, defaulting to plain text.