Add an option to remove the !m sigil from the plaintext too

[mattiklock]

I basically applied this patch to muttdown locally:

c72c5fe

Thank you for all of the recent updates!

[zacchiro]

Thanks for drafting a patch for this, which is indeed very useful.
Regarding the proposed solution, though: is there a reason to actually not remove the sigil from the plain text payload? I think it should always be removed, as it's done for the html payload.