Adding support for direct byte-buffers

I really like the synergy between `byte-streams` and `gloss`, so I found it a bit odd that while `byte-streams` supports converting to a DirectByteBuffer via the options map, there doesn't seem to be way for `gloss` to create DirectByteBuffers from its codecs. Perhaps `encode` could accept an optional option map similar to how `byte-streams/convert` does?
