* Add initial support for getting ChannelMember info of all bridges.
Adds an EventGetChannelMembers event, which gets send every x time to
all bridges. Bridges should respond on this event with a Message
containing ChannelMembers in the EventGetChannelMembers key in the
Extra field.
handleEventGetChannelMembers will handle this Message and sets the
contained ChannelMembers to the Bridge struct.
* Add ChannelMembers support to the slack bridge