go: use a different goconfig package

The notmuch-addrlookup utility uses a third party library to read the
notmuch configuration file. The previously used implementation at
"github.com/kless/goconfig" vanished, so this patch switches to the
implementation at "github.com/msbranco/goconfig". As the
implementations differ at the API level, the code is updated
accordingly.

Signed-off-by: Justus Winter <4winter@informatik.uni-hamburg.de>