Index user defined headers

[David Bremner]

This obsoletes [1]. Compared to the previous version the main change
is that it imposes the restriction that user defined prefixes may not
start with [a-z], and must consist of "unicode word characters". This
assumes a utf8 input encoding. People that don't like utf8 are welcome
to use ASCII :P

[1]: id:20190302154133.25642-1-david at tethera.net
[2]: https://salsa.debian.org/bremner/notmuch/commits/wip/user-headers