Options in data-setup attribute are ignored when the initialization is called before autoSetup

Because only `vjs.autoSetup` picks the option from `data-setup`, this attribute will be ignored if the initialization is called before that. Sometimes I face this issue and options in `data-setup` are ignored, so I made `Player.getTagSetting` pick the option from `data-setup` as well.
If we're doing this here now we should also remove it from the autoSetup script. And move the code comments over from there over to this one. Just the "If empty string, make it a parsable json object" one.
https://github.com/videojs/video.js/blob/b65f2da65e848cd5a6e53debd295565de7e25067/src/js/setup.js#L31