Add support for metadata_specifier

[stonio]

Add support for argument -metadata[:metadata_specifier] key=value.
See https://www.ffmpeg.org/ffmpeg-all.html#Main-options

[bramp]

We support: -metadata key=value via the FFmpegOutputBuilder.addMetaTag(..), but not the metadata_specifier. I will look at adding.

[bramp]

ok, I'm thinking something like:

addMetaTag("key", "value", global());
addMetaTag("key", "value", chapter(1));
addMetaTag("key", "value", program(1));
addMetaTag("key", "value", stream(0));
addMetaTag("key", "value", stream(VIDEO, 0);
addMetaTag("key", "value", stream(AUDIO, 0);
addMetaTag("key", "value", stream(id(0x502));
addMetaTag("key", "value", stream(tag("key2", "value2"))); // This one is confusing, key2 refers to the output stream, and would apply key/value to any output stream already with key2/value2.