library config API

[David Bremner]

This is the third revision of this series, and I think the first two
patches are ready for serious review.

Some things to think about with respect to the CLI patches

- Should we include a "version" line at the top of the dump? Currently this is unused, but that
  depends on the format being auto detectable?

- Should config info be included by default in the dump output?

- Should we restore config info by default?

- How about the usual bikeshedding about argument names? I'm loath to
  spend time on docs until after this.

I guess a global question is what do we need this API / feature for. I
can think of three things so far

- tag aliases [1]

- making the location of the mail tree configurable (i.e. allowing
  mailtree and database to live in different places).

- Some things like excludes and maildir synchonization are natural to
  have in common between all clients.

A natural question is why not provide a library API to access
.notmuch-config? Having a plain text representation of the
configuration (without the extra step of dumping it) is obviously
attractive, but

- We would need to build some locking / synchronization to prevent
  various clients and interactive text editors from stomping on each
  other. This we have for free by using xapian metadata.

- We'd also have to provide our own key/value datastructure.

- At least for tag aliases, I think there is a second argument that
  this information part of the users tagging state, and should be
  backed up as part of the normal dump / restore process.

[1]: id:87mvsd7cxr.fsf at zancas.localnet