emacs: Allow custom tags formatting

emacs: Allow custom tags formatting

Previously we ignored any notmuch-search-result-format customizations
for tag formatting because we needed to be able to parse back in the
result line and update the tags in place.  We no longer do either of
these things, so we can allow customization of this format.

(Coincidentally, previously we still allowed too much customization of
the tags format, since moving it earlier on the line or removing it
from the line would interfere with the tagging mechanism.  There is
now no problem with doing such things.)