Implement further ZLIB flush modes.

[oyvindln]

Using flush on the writer currently ends the compression stream and writes a trailer (without resetting the encoder.) The writers in flate2 calls miniz/zlib with SYNC_FLUSH, which outputs the current pending data and adds an empty block at the end. This is probably the behaviour we should emulate.

[oyvindln]

Sync flush has been implemented, but I'll leave the issue open for now.

[oyvindln]

May want to add support for other methods and an explicit writer method to flush.