Add Brotli as Sparkle compression option

[AndrewSimard]

Brotli Compression

Not sure if this should be posted here or in Sparkle, but as the middleware is added to the XDataServer component, maybe here makes the most sense. I'd like to see a new compression option for Brotli. This is something that I think is now supported by the majority of browsers and offers better compression than gzip. I've been experimenting with it a bit using the YuBrotli code to compress files containing JSON. Using gzip, the files were about half the size. With Brotli (max compression) th…