Add support for environment variable $BUGWARRIORRC
* If `$BUGWARRIORRC` is set, then bugwarrior will try to read its
configuration file from its content
* Extract the code that guess the path to the configuration file in its
* Add some unit tests
I got a bit tired of editing my own `.bugwarriorrc` to test bugwarrior. This is inspired by the similar variable `TASKRC` used by taskwarrior.
Looks good here. Thanks for hacking on this @gdetrez! :+1: to merge from me.