add delete keybinding

[Dirk Hohndel]

Straight forward addition to the Emacs UI. The 'd' keybinding
is implemented very similar to the 'a' keybinding - it only
adds a +deleted tag as well. This tag is used by notmuchsync
to delete (-p "prune") files in the mailstore.

I'm sending this mostly as an RFC - I use this and like it, but
people seem to have strong feelings as to how they want to deal 
with deleting email (or for some people, how they don't want to
do that at all).

/D