Initial support for selecting what is output

[david at tethera.net]

These are some preliminary patches to add a --output argument to
notmuch show.

	notmuch show --format=json --output=from,subject ${query}

or

	notmuch show --format=json --output=to,body ${query}

I'd like to implement a bit finer control (to get message-id's only is
not possible right now), and it needs documentation and a test, but I
thought I'd toss this out there to see what people think.