Support for brotli as response content-encoding and request body compression

[na--]

As seen here, there's some usage of brotli and it might be a good idea to support it.

Original library by Google: https://github.com/google/brotli

There are a bunch of bindings to it, but the only in pure Go I've found with a brief search is this one: https://github.com/dsnet/compress/tree/master/brotli