Change avoptions_non_linux.jl -> avoptions_ffmpeg

* Added `@ffmpeg_only` and `@libav_only` macros
* Allows the relevant code to run on linux when using ffmpeg libraries

Sending pull requests directly to you should make it easier for you than having you rebase off of master.

I'll probably make a few more with suggested changes.

Since you have push access to my VideoIO repo, in the future you can create/push directly to a branches there, which will make it easier for me to push changes.  (This is common in mainline Julia.)