Add bindings for flac 1.2.1.

Here's the bindings for flac 1.2.1.

I don't appear to have any commit access to the D-Programming-Deimos/flac repo, so I can't create any branches for it or push these changes myself, but I'd appreciate it if whoever merges this could then branch it as 1.2.1. After that, I can create another pull request that includes the bindings for flac 1.3.0.