WIP2: index user headers

[David Bremner]

David Bremner <david at tethera.net> writes:

> I had another thought about user prefixes. I wonder if they should all
> be forcibly prefixed with something that prevents collisions, to prevent
> later pain if we add an "official" prefix with the same name. A quick
> tests suggest it would work to use something like _
>
> so
>
>         notmuch search --output=files _list:notmuch
>
> works. It's a bit ugly, I'll have to play with other options; the main
> question is whether we think prefixing is needed / worth-it.

I played with the query parser a bit, and the only idea I found so far
is to reserve prefixes starting with lower case ASCII for notmuch, and
allow users to use anything else.

d