Netflix: check DOM for ".image-based-timed-text" to warn no support for Chinese & Japanese

[mikesteele]

Related to issue #20

Netflix uses images for Chinese and Japanese text and there should be a adaptor method to check the DOM for the presence of elements to warn the user of no support. The method should be run outside of the caption lifecycle, perhaps on popup opened. (on App's componentDidMount)

[mikesteele]

Translation note: it's possible that other languages use image-based-timed-text, so the error message should be general. Eg. "Netflix uses images to render captions for languages like Chinese and Japanese, so we can't translate the captions currently on the page. YouTube will work. See issue #20.

[mikesteele]

Same for #41