Re-wrote the middleware compress library.

[awbraunstein]

This re-write introduces the concept of a Compressor which stores all the settings for how compression/encoding should happen. The old API was changed to use the new Compressor struct which should prevent this change from breaking current consumers.

Additionally, this uses a sync.Pool for encoders that have a Reset(io.Writer) method to reduce memory overhead.

The new API is as follows:

r := chi.NewRouter()
compressor := NewCompressor(5, "text/html", "text/css")
// Add in a no-op encoder.
compressor.SetEncoder("nop", func(w io.Writer, _ int) io.Writer {
	return w
})
r.Use(compressor.Handler())

This fixes #397.

[pkieltyka]

wow @awbraunstein well done. the code looks very sharp and complete. I will review it in detail this evening. thanks for the quick turnaround

[pkieltyka]

did a review, tested various edge cases manually, and works great. thanks again @awbraunstein

[VojtechVitek]

Sweet contribution! Thanks ❤️