implement GetCaption and SetCaption for HTMLTableElement

[ajnirp]

Fixes #3052

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @kmcallister (or someone else) soon.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2373

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[Manishearth]

Merged, congrats on your first patch!

If you want something similar, try out createCaption() and deleteCaption(), or some of the other table methods. There are also some "less easy" bugs here, though many of those might require python codegen changes.

Next time, don't do that merge in the middle of the review. ;)