Add ability to specify feed's group when creating a new feed

[brentru]

As of now, feeds created will be created in the "default" group.

Instead of creating feeds in the default group, change the create_feed call to include a group kwarg:
def create_feed(self, feed, group=None)

Group formatting: https://io.adafruit.com/USERNAME/feeds/GROUP.FEED

Related thread: https://forums.adafruit.com/viewtopic.php?f=56&t=156692

[Thuzerland]

Is there any progress?

[brentru]

Closing via #125