long life to notmuch-mutt

• Bad news: I've just killed mutt-notmuch
• Good news: mutt-notmuch has been integrated upstream as a notmuch contrib script under the name notmuch-mutt
  (note the word switch, due to uniformity with other tools of the notmuch suite)
• Good news (for Debian users): starting with next notmuch upload there will be a Debian notmuch-mutt package. It will work out of the box with Mutt, without requiring any ~/.muttrc fiddling

As a consequence, no further separate releases of mutt-notmuch will be made. Future releases (of notmuch-mutt) will happen as part of notmuch.

In other news, several changes have been implemented in notmuch-mutt wrt mutt-notmuch 0.2:

• move cache dir to ~/.cache/notmuch/mutt/ by default
  (Note: given that, from the point of view of notmuch, the next one will be the first release, no migration code from the previous cache location is present. Please switch to the new version and get rid of ~/.mutt-notmuch* manually.)
• support for XDG basedir, allowing to override the cache dir
• switch to String::ShellQuote for proper shell escaping in all notmuch calls
• system-wide Mutt configuration snippet for /etc/Muttrc.d/ (Debian-specific) or /etc/Muttrc

Many thanks to David Bremner (for shepherding my changes in) and to Ben Boeckel (whose desire to package mutt-notmuch in Fedora made me kick Debian bug #628018, this time for real).