Add brotli support

[tracker1]

Closes #21, full version bump because of behavioral change, still backwards compatible though.

Only enabled brotli if the optional dependency of iltorb is available and br option is specified. Added notes in the readme, as well as a suggestion to only use in conjunction with an output cache.

In my own tests:

encoding  level/quality  time   size
gzip      9              36ms   129kb
br        11             441ms  127kb
br        8              43ms   128kb
br        6              38ms   128kb
br        5              36ms   128kb

[codecov]

Codecov Report

Merging #80 into master will decrease coverage by 7.31%.
The diff coverage is 91.89%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage     100%   92.68%   -7.32%     
==========================================
  Files           1        1              
  Lines          32       41       +9     
  Branches       15       17       +2     
==========================================
+ Hits           32       38       +6     
- Misses          0        3       +3

Impacted Files	Coverage Δ
index.js	92.68% <91.89%> (-7.32%)	⬇️

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[tracker1]

Looks like travis borked, and not sure about updating the tests... I implemented out of my own curiousity.

[curbengh]

brotli support has been added to node >= 10.16. native brotli can be prioritized over iltorb when supported.

See expressjs/compression#158 expressjs/compression#156

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Just stumbled upon https://github.com/tuananh/kompression which is a fork with brotli support.

[jonathanong]

@tracker1 sorry about the delay. going in favor of #92 since it is using native node APIs. please review it!

[tracker1]

@jonathanong that's cool... my PR predates native support iirc... I did follow the other PR, and lgtm.