Add parameter for set maximum length after decompress

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: https://github.com/aio-libs/aiohttp/issues/8248