Add an API to set caption styles

[joeyparrish]

The FCC has guidelines for closed captioning in the US that in some cases apply to internet streaming. (I am not a lawyer and do not pretend to understand who is subject to the rules when and why.) In particular, the user must be able to control font size, font color, font edge effect, background color, and opacity. They also specific specific values that a user must be able to choose from for each of these properties.

Service providers can already comply with this using JavaScript and CSS. This API would simply make it easier by defining an easy and clear mechanism to control caption styles. It could also be useful for app developers who simply want to offer more control over captions as part of their user experience.