Implement http_brotli_static module

Today, nginx supports the http_gzip_static module ( for doing ahead-of-time gzip compression of assets; this is very useful because it allows using high-ratio/low-performance compressors like Zopfli.

Web Browsers (Chrome and Firefox, to start) are about to start supporting a new Content-Encoding, named Brotli, which offers MUCH improved compression ratios but with slow performance. As such, we need nginx to support a similar module (e.g. http_brotli_static) for this new content-encoding.