feat(writer): Allow getting underlying writer of ZipWriter

[roblabla]

These allow accessing the underlying writer, which can be useful to read out the underlying state, for instance to send it out.

In my case, I need to zip chunks to a Vec, that will be periodically (after X writes) be sent out to the network and replaced. We have to work this way due to API constraints.

Most of the compression crates out there already offer support for getting the underlying writer (bzip2, zstd, flate2).

Note: this depends on:

• get_ref/get_mut in zopfli-rs: Add DeflateEncoder::get_ref/get_mut zopfli-rs/zopfli#69
• get_ref/get_mut in ppmd-rust: Add Ppmd8Encoder::get_ref/get_mut. hasenbanck/ppmd-rust#10

[AlexTMjugador]

I just noticed this PR was created, so I've just published a new zopfli release to make the required change available without dependency patching 🙂