support strict mode

[gkatsev]

Right now, this breaks when used in strict mode because in createElement it tries to set localName back to the tagName but setting localName on the element isn't allowed in strict mode. Loose mode lets it slide, though.
Making a change to video.js to disable strict mode for now (videojs/video.js#4551) but it should be fixed here ultimately.

[gkatsev]

And videojs/video.js#4551 should be reverted when this is fixed.

[gkatsev]

#40 fixes this. We should revisit and revert the video.js change when updating to vtt.js 0.15.0.