Add parameter for set maximum length after decompress

[ikrivosheev]

Hello! It'll be great to have parameter max_length in Decompressor (http://python-hyper.org/projects/brotlipy/en/latest/api.html#brotli.Decompressor.decompress) like in python zlib: https://docs.python.org/3/library/zlib.html#zlib.Decompress.decompress.

The parameter mean that the return value will be no longer than max_length. I need it for use in http client.
Related issue in aiohttp: aio-libs/aiohttp#8248