lib/config: add notmuch_config_key_{get,set}

By using an enum we can have better error detection than copy pasting
key strings around.

The question of what layer this belongs in is a bit
tricky. Historically most of the keys are defined by the CLI. On the
other hand features like excludes are supported in the
library/bindings, and it makes sense to configure them from the
library as well.

The somewhat long prefix for notmuch_config_t is to avoid collisions
with the existing usage in notmuch-client.h.