hlsjs: checks if MSE is enabled
hlsjs: checks if MSE is enabled

Why do we need to explicitly skip Safari? (isn't [html5 video](https://github.com/clappr/clappr/blob/master/src/components/loader/loader.js#L47) playback [able to play](https://github.com/clappr/clappr/blob/master/src/playbacks/html5_video/html5_video.js#L267) it?)

@cpeterso is safe to assume that `!!window.MediaSource` will return `false` for all FF with disabled MSE?
dayvson/clappr,clappr/clappr,dayvson/clappr,flavioribeiro/clappr,clappr/clappr,clappr/clappr,flavioribeiro/clappr,dayvson/clappr,flavioribeiro/clappr