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

[Ciprian Dorin Craciun]

According to the `devel/schemata` the message object doesn't contain
the thread identifier to which it was assigned in the database.

Sometimes, for example in an UI that displays a search result at
message level, it would be useful to know the thread each message
belongs to, so the user can easily switch to the entire thread.

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

Ciprian.