notmuch show: Add a --format=mbox option

notmuch show: Add a --format=mbox option

We don't love the mbox format, but it's still sometimes the most
practical way to share a collection of messages as a single file.

Here we implement the "mboxrd" variant of the mbox file format. This
variant applies reversible escaping by prefixing a '>' character to
all lines in the email messages matching the regular expression:

	"^>*From "

This allows the escaping to be reliably removed. A reader should remove
a '>' from any line matching the regular expression:

	"^>>*From "

More details on the mboxrd formats (and others as well) can be found
here:

http://homepage.ntlworld.com/jonathan.deboynepollard/FGA/mail-mbox-formats.html