emacs: set default in notmuch-read-query

emacs: set default in notmuch-read-query

This adds the current query as a "default value" to
notmuch-read-qeury. The default value is available via a down-arrow as
opposed to history which is available from the up arrow.

Note if a user presses return in the minibuffer this value is not
returned.

The implementation is simple but notmuch-read-query could be called
via notmuch-search/notmuch-tree etc from any buffer so it makes sense
to put the decision of how to extract the current query in
notmuch-read-query rather than in each of the callers.