Currently fully support mattermost,slack and discord.
Message deleted on the bridge or received from other bridges will be
deleted.
Partially support for Gitter.
Gitter bridge will delete messages received from other bridges.
But if you delete a message on gitter, this deletion will not be sent to
other bridges (this is a gitter API limitation, it doesn't propogate edits
or deletes via the API)
* Move from gcfg to toml configuration because gcfg was too restrictive
* Implemented gateway which has support multiple in and out bridges.
* Allow for bridging the same bridges, which means eg you can now bridge between multiple mattermosts.
* Support multiple gateways
