Add support for plain text, html and multipart emails in TemplatesPlugin

The templates plugin already allows to return customized content for the emails. Though, the email content is already sent as content type html.

This change improves it to allow sending only text, only html or multipart emails. The templates plugin now returns a tuple of Option[Txt] and Option[Html], so that clients can choose which parts they want to provide.

As the mailer plugin handles null or empty content gracefully we can reduce None simply to null or an empty string (the latter is chosen because the mailer plugin also passes an empty string if a part shall be omitted).

Closes issue #128