Most of the infrastructure here is general, only the validation/dispatch
is hardcoded to a particular prefix.
A notable change in behaviour is that notmuch-config now opens the
database e.g. on every call to list, which fails with an error message
if the database doesn't exit yet.
Previously we set up a way for the top level notmuch command to choose
which gpg binary was invoked by libgmime. In this commit we add the
(mostly boilerplate) code to allow the notmuch-config command to read
and write this path, and use it in the appropriate struct.
Update tests for new default variable
This is the output from sphinx-quickstart, massaged a bit, along with
our existing man pages converted to rst.
A skeleton notmuch-emacs manual is also included. It is not suitable
for end user use yet.
