Support optional x264 dependency

[haileys]

This PR adds optional support for x264. It's disabled by default for licensing reasons, so users of this crate must opt in to x264 support by enabling the gpl and x264 features.

This pull request depends on the pkgconfig manifest PR over at https://github.com/imager-io/x264-dev/pull/1

[colbyn]

Interesting, initially when I wrote this library it was for the purpose of container muxing/demuxing and general decoding, and so in conjunction with keeping it self contained I never thought about supporting x264 for AVC encoding. Also seemed like I would be implementing the build stuff twice rather than use the x264 crate as you proposed.

Nice work :)