Add missing elements that have a factor on contrast

[mfairchild365]

document.elementsFromPoint does not include the TBODY and THEAD elements, so contrast was not being calculated correctly under some circumstances involving those elements. See added tests for examples.

[dylanb]

Looks good to me - thanks for the contribution!!!

[dylanb]

@mfairchild365 the changes are good but I just noticed that the commit messages do not match our required style (which allows us to generate our release notes). Could you fix this using rebase?

[mfairchild365]

@dylanb rebase was giving me grief, so I just deleted the branch, added the changes back, and made a new commit.

[dylanb]

Awesome!! @WilcoFiers @marcysutton any further comments?

[marcysutton]

Thanks so much for the contribution @mfairchild365! I ran the tests on this branch since Circle still isn't running on forks (#497) and they look all good.

It seems like a good addition to me; I wish we didn't have to do so much to shim support for document.elementsFromPoint but I'm glad we already had logic in place to expand upon. I didn't even know you could style those elements, TIL! :)

[marcysutton]

I'll make a few changes so this can go into develop-2x as well.