notmuch show: Switch to control character to mark sections of output

We were previously using things like "%message{" which were not
guaranteed to never appear in an email message. Using a control
character (^L or '\f' instead of '%') gives us better assurance that
our delimiter doesn't show up in an original email message.

This still isn't entirely safe since we're decoding encoded text in
the body of the email message so almost all bets are off really.