#495 improve handling of malformed HTML-only emails

[Mike Kupfer]

This is in response to recent discussion on mh-e-users.

Some mail programs generate HTML email without putting the appropriate metadata (e.g., "Content-type: text/html") in the email header. So MH-E treats the message body as plain text, and the user is left to decipher HTML.

Ticket #158 describes one approach to deal with this: make it easy for the user to send the HTML to an external viewer.

Another approach would be to provide an easy way for the user to tell MH-E to treat the message as HTML. This would be particularly useful if the user uses shr, rather than an external program, to view HTML.

Even more convenient for the user would be for MH-E to automatically recognize that the body is HTML and treat it appropriately. In that case, MH-E should also provide a way to turn this off, so that the message is treated as simple text.