Provide context for altering messages

I would like to be able to alter my message body to get around a limitaiton with the HTML formatting, but since there is no context available I am unable to do so. The attached patch passes in several contexts for the message: $params which are the results of the views_send forms, $token_data, and $views_send_token_data so I can re-do the token replacements.

This patch also includes a fix to support the combination of the mailsystem and htmlmail modules, which was not previously possible. Please Let me know if you would prefer that be done in a separate issue.