text: Title: DataChannel-only 'feature' without codec dependencies?
  Body: Great stuff! I'd love to use this in [magic-wormhole](https://github.com/warner/magic-wormhole).. I only need the DataChannel, though. Any idea how hard it'd be to define a 'feature' for the codecs, so `pip install aiortc[video]` gets you the dependencies on the codecs, but `pip install aiortc[datachannel]` does not?
  