Add rule(s) for HTTP compression

[alrra]

gzip

1. Check is something is served compressed using gzip:

   Make a request with the Accept-Encoding: "gzip" header, then verify if the response is served with the Content-Encoding: "gzip" header and the body of the response starts with 1f 8b.

2. What should be compressed with gzip:

   File type	Commonly used file extension(s)	Commonly used media types(s)
   Atom	.atom	application/atom+xml
   App Cache Manifest	.appcache	text/cache-manifest
   BMP	.bmp	image/bmp
   CSS	.css	text/css
   Cursors Images	.cur	image/x-icon image/vnd.microsoft.icon
   Embedded OpenType font	.eot	application/vnd.ms-fontobject
   Favicon	.ico	image/x-icon image/vnd.microsoft.icon
   HTML	.html .htm ...	text/html application/xhtml+xml
   HTML Components	.htc	text/x-component
   JavaScript	.js	application/javascript text/javascript
   JSON	.json ...	application/json application/<something>+json
   OpenType font	.otf	font/opentype
   RDF	.rdf	application/rdf+xml
   RSS	.rss	application/rss+xml
   Source Maps	.map	application/json
   SVG	.svg	image/svg+xml
   TrueType font	.ttc .ttf	application/x-font-ttf
   TXT	.txt	text/plain
   vCard	.vcard vcf	text/vcard
   VTT	.vtt	text/vtt
   XML	.xml ...	application/xml text/xml application/<something>+xml
   Web App Manifest	.webmanifest .json	application/manifest+json

   Notes:

   • Under 1K things should not necessarily be compressed.
   • WOFF fonts should not be compressed (see: Serve WOFF 1.0 font files compressed? h5bp/server-configs-apache#42).
   • SVGZ should be served with the Accept-Encoding: "gzip" header as they are compressed by default.

Zopfli

1. Check is something is served compressed using Zopfli:

   Same as with gzip, just that we need to detect Zopfli.

2. What should be compressed with Zopfli:

   Same as with gzip.

   Note(s):

   • We will need to a check if WOFF fonts use Zopfli compression internally.

Brotli

1. Check is something is served compressed using Brotli.

   Make a request with the Accept-Encoding: "br" header, then verify if the response is served with the Content-Encoding: "br" header (no magic numbers?).

   Note: Brotli compressed responses should be served only over HTTPS.

2. What should be compressed with Brotli:

   Same as with gzip.

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Note: This is just a starting point, we will probably split this into more specific issues / rules.

[molant]

This has a dependency on #164 to work cross collector.

[AndrewCraswell]

+1 for this. Especially the highlighting of Brotli compression!

[molant]

@alrra is working hard on this