Dedicated decompressors

Adds uWS::CompressOptions::DEDICATED_DECOMPRESSOR. This (decompressor) flag can be OR:ed with one (compressor) flag to easily create a complete compression preference such as:

.compression = uWS::CompressOptions(uWS::DEDICATED_COMPRESSOR_4KB | uWS::DEDICATED_DECOMPRESSOR),

See #1347. This change is backwards compatible. More specific decompressors will be added with time.