Add support for group_filter option on z2m server blocks

[chrivers]

Bifrost presents z2m groups as Rooms, but users might not want to present every single group in bifrost.

To support this, add "group_prefix" option to z2m server blocks, like so:

...
z2m:
  myserver:
    url: ws://10.10.0.102:8080
    # Group prefix [optional!]
    #
    # If you specify this parameter, *only* groups with this prefix
    # will be visible from this z2m server. The prefix will be removed.
    #
    # So with a group_filter of "bifrost_", the group "bifrost_kitchen"
    # will be available as "kitchen", but the group "living_room" will
    # be hidden instead.
    group_filter: bifrost_

[chrivers]

Implements idea mentioned by @joeblack2k in #6