second attempt: JSON based search

[David Edmondson]

Here's another attempt that uses the proper JSON output directly -
there is no requirement to either modify that output (as previously)
or add another similar output format (as discussed on IRC).

[PATCH 1/3] notmuch: Add `date_relative' to JSON search output.
[PATCH 2/3] test: Expect `date_relative' in JSON search output.
[PATCH 3/3] emacs: Use JSON output for search.