Provide a way to test whether a stream is flushable

[nemequ]

Some codecs can flush (zlib, bzip2, lzma, …), most can't. Just like squash_codec_knows_uncompressed_size tests for whether or not the uncompressed size can be extracted from a compressed buffer without decoding the whole thing, there should be a squash_codec_can_flush (or similar) function to detect whether or not a codec knows how to flush.