[dom] Added an example showing feature detection with supports() (#144)

In a [PR](https://github.com/w3c/preload/pull/39) for an example of DOMTokenList's supports() based feature detection, @igrigorik suggested that it might be better that the example will live in a more central location. Therefore, I think it would make sense to add it in DOM.

@annevk - can you take a look?
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/144

-- Commit Summary --

  * Added an example showing feature detection with supports()

-- File Changes --

    M dom.bs (22)
    M dom.html (307)

-- Patch Links --

https://github.com/whatwg/dom/pull/144.patch
https://github.com/whatwg/dom/pull/144.diff

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https://github.com/whatwg/dom/pull/144