expose "write_chunk" functionality

[JasperDeSutter]

The current API does not allow writing arbitrary chunks. This is because the Writer is not exposed on the Encoder struct, and the writer module is not public.
This is currently limiting me from using mtpng, as I need to be able to write extra/custom chunks other than allowed currently.
Maybe a new write_chunk method could be added to the Encoder that simply forwards to the inner Writer?

[bvibber]

Yes, something like that should be perfect for custom chunks. I'll take a peek at the code soon and see if I can hack it in. :)

[bvibber]

0.3.4 (5318501) should resolve this, let me know if it's working for you!

[JasperDeSutter]

Thanks for adding this feature so fast! It's working perfectly fine for my needs.