Allow sending HTML-only emails

Currently messaging_phpmailer.module uses PHPMailer's AltBody parameter to automatically include both HTML and plain text MIME parts.

In our use case, the plain text emails are never very useful since our HTML emails include significant formatting, so the plain text version just ends up wasting bandwidth.

The attached patch adds an option to messaging_phpmailer.module's Send Methods configuration form, allowing the user to specify that HTML-only emails will be sent.