Brotli support

[daurnimator]

https://tools.ietf.org/html/rfc7932

Possibly using https://github.com/witchu/lua-brotli or https://github.com/soulik/luabrotli

Related to #22

Note that some browsers only allow brotli for HTTPS due to broken proxies (see https://samsaffron.com/archive/2016/06/15/the-current-state-of-brotli-compression).

[daurnimator]

None of the lua brotli libraries I found currently have a streaming interface.