Index user defined headers

[David Bremner]

David Bremner <david at tethera.net> writes:

> 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

Last call for review or other feedback. I'll merge this in the next week
if I don't hear anything.

d