This patch allows the notmuch backend to retrieve the notmuch database p...

...ath from the notmuch configuration file in case no "db_path" argument is passed to the Notmuch constructor.

In such a case, i3pystatus will try to load the file designed by the
environment variable NOTMUCH_CONFIG and "~/.notmuch_config". and
retrieve the value "path" of the section [database].

I don't handle the errors such as "section database doesn't exist" as i3pystatus policy looks to be minimal checking. There might be a case where NOTMUCH_CONFIG file actually exists and is different from an existing ~/.notmuch_config. The value of .notmuch_config may be taken instead of the one from NOTMUCH_CONFIG but I am not sure it's worth treating.