Added feature to allow coerion to plaintext for html messages

For 5 years (since I've been using android) I've been waiting for an email client which allows me to read and write email in plain text only (as in Thunderbird, on the gui or as in many email clients over tty).

K9 seemed to be the closest by allowing me to only compose in plain text and to use monospace font when viewing text/plain messages.  However, even in K9 it seems that html was always preferred.

This feature was requested multiple times in multiple issues threads, a quick search turned up these: Issue 1714, Issue 2132, Issue 6079, Issue 4456.  My implementation was probably rudimentary and possibly buggy, but it got me what I wanted and I'm now using it on all my devices so I thought I'd issue a pull request in case it's helpful for others.

I love k9, now that it has this last feature from my fork, it does everything I want from an android email client.

Thanks for all the hard work!