Add support for `thread` field in `notmuch show`

[David Bremner]

Ciprian Dorin Craciun <ciprian.craciun at gmail.com> writes:

> I know that one can use `thread:{id:MESSAGE_ID}` to achieve the same
> result, however:
> * it is somewhat cumbersome for the integrator;

Out of curiousity, what is harder about it? In both cases you have to
extra one value from the JSON.

> * having the thread identifier explicitly, could be used as a key in a
> cache, or other internal lookups;
>
> In fact the only way one can extract the thread identifier via the
> `notmuch` CLI is to use `notmuch search --output=threads --
> id:MESSAGE_ID`

Offhand I have no strong objection to someone (who is not me) adding
this. I think it's important to be aware that thread id's are ephemeral,
and subject to change e.g. if the database is re-built from
scratch.