Support brotli decoding

[hrj]

Firefox 44 will have brotli decoding, for https requests only.

In order to keep our Accept header the same and to benefit from better compression, gngr can also support brotli.

[hrj]

A test image: https://www.bayden.com/test/brotliimg.aspx

An implementation in rust: https://github.com/ende76/brotli-rs