COMPRESSOR_CLASSES config updates default compressors instead of replacing

[alanhamlett]

This makes adding custom compressors simpler. Instead of having to list all default compressors, your Flask config can just list custom ones.

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This change is 

[jaysonsantos]

Thanks again @alanhamlett. Could you take a look on the tests? They broke because they could not find the executables to compile the files.

[jaysonsantos]

Hi fixed it in this commit dcee092 I will merge #55 on master that has it, can you merge it? I am afraid that there will be conflicts and/or your branch will break because i put lint and isort checks

[jaysonsantos]

Thanks again @alanhamlett I will bump and release a new version