python: add bindings to access config

The C functions notmuch_database_get_config,
notmuch_database_get_config_list and notmuch_database_set_config are
part of the official C bindings.  So there should also be some python
bindings for them.

Also they are the only way to access the named queries introduced in
b9bf3f44.

The interface of the python functions is designed to be close to the C
functions.