NEWS: add news for 0.18.1

This is my summary of the commits from 0.18.
version: bump to 0.18.1~rc0

Also bump the python bindings version, the NEWS version and the Debian
version.

Since the changelog is (slightly dubiously) metadata, we have to
change it to upload a release candidate.
Merge tag '0.18.1_rc0'

notmuch 0.18.1~rc0 release