Support linking with a subset of FFmpeg using feature flags

[JohnstonJ]

My application has no use for some FFmpeg libraries. For example, I don't need to use avdevice because I'm not working with any actual devices - I'm just transcoding media files. So I built FFmpeg without that library. (I am using vcpkg and they offer a feature for each FFmpeg library that you want to build, so I excluded avdevice.)

rust-ffmpeg-sys is a similar type of package offering FFmpeg bindings, and it offers the ability to opt in & out of each library using features: https://github.com/zmwangx/rust-ffmpeg-sys/blob/a13516c4f2183330468f3f5867262a78674f61e0/Cargo.toml#L103-L111

By excluding some features, it reduces the build time for building FFmpeg. And less things for the compiler/linker to deal with: a whole bunch of bindings don't even need to be generated, etc.

My current preference is rsmpeg over ffmpeg_next, but it would be nice if I could build only the subset of FFmpeg that I need.