cli: add insert command

The notmuch insert command reads a message from standard input,
writes it to a Maildir folder, and then incorporates the message into
the notmuch database.  Essentially it moves the functionality of
notmuch-deliver into notmuch.

Though it could be used as an alternative to notmuch new, the reason
I want this is to allow my notmuch frontend to add postponed or sent
messages to the mail store and notmuch database, without resorting to
another tool (e.g. notmuch-deliver) nor directly modifying the maildir.