Bridge gitter and IRC

[patcon]

Re-ticketed from https://github.com/chaijs/chai/issues/1038#issuecomment-379567686

cc: @shellscape @keithamus

To Do

• [ ] @patcon creates bridge on an existing instance for evaluation
• [ ] Update Community section to explain. (prior art)
• [ ] Evaluate for X weeks
• [ ] Community hosts own bridge (optional?) -- using matterbridge-heroku as template