feat(middleware): add Deflate middleware

[masakichi]

Add Deflate middleware which is very similar to the current Gzip middleware's implementation, I think it will be nice to have deflate as well if someone also needs it.

I didn't change any public function and struct in compress.go, and didn't touch existing test cases in compress_test.go as well, so I think this PR won't break backward compatibility.

[codecov]

Codecov Report

Merging #2085 (6e6ae1e) into master (4a1ccdf) will decrease coverage by 0.02%.
The diff coverage is 88.00%.

@@            Coverage Diff             @@
##           master    #2085      +/-   ##
==========================================
- Coverage   92.21%   92.18%   -0.03%     
==========================================
  Files          37       37              
  Lines        3019     3034      +15     
==========================================
+ Hits         2784     2797      +13     
- Misses        148      150       +2     
  Partials       87       87              

Impacted Files	Coverage Δ
middleware/compress.go	84.84% <88.00%> (+0.84%)	⬆️
ip.go	100.00% <0.00%> (ø)
echo.go	95.14% <0.00%> (ø)

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