text: Title: Make media codecs optional
  Body: For some use-cases, I think that media codecs are not required. For example, I am just interested in data channels.
  
  Would you accept a PR that moves `av` to [extra_require](https://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies) and make the `mediastreams` module optional?