adding a currentType method to get current source type if known

Alternative solution for the problem posed in #1313. See that PR for discussion.
I just realized the isReady check here (which existed previously) is redundant. It's doing the same as the [ready function in Component](https://github.com/videojs/video.js/blob/master/src/js/component.js#L616), so you could just always call the ready function, and that might simplify this a little.