Add support for the `track` element

[whisk]

Description

The library currently lacks support for the track element. HTML5 reference: https://html.spec.whatwg.org/multipage/media.html#the-track-element. Note that this element has no end tag, that means it may not have children.

What to do

• [ ] Add new element function in elements.go
• [ ] Add the test case in elements_test.go
• [ ] Check attributes which the track element has, add them to attrs package if not already present.