Support .flush for Compressor

[milesgranger]

#64 introduced streaming input, but of course, most will also want streaming output. :-)
Continues to follow the brotli API, such that flush will return what is currently compressed by the encoders and truncate its buff.

ie.

>>> compressor = cramjam.snappy.Compressor()
>>> compressor.compress(b"foo")
>>> out = bytes(compressor.flush())
>>>
>>> compressor.compress(b"bar")
>>> out += bytes(compressor.flush())
>>>
>>> out += bytes(compressor.finish())
>>> decompressed = cramjam.snappy.decompress(out)
>>> assert bytes(decompressed) == b"foobar"

ref: developmentseed/starlette-cramjam#1