added brotli compression support

[mpf82]

brotli compression can be enabled just like gzip compression by setting 'tools.br.on': True,

tools.br may also be combined with 'tools.gzip - the client can then decide which encoding is accepted.

If both gzip and br tools are enabled and both gzip and br are accepted encodings, br will be chosen over gzip - unless gzip has a higher qvalue

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