Add HTML aria attribute autocomplete

[Tyriar]

Fixes #320

/cc @aeschli

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[dstorey]

Should this take into account the aria attributes are not really global? Some are only valid if the element is a particular role (either via role attribute or the element's natural semantics)

[Tyriar]

That's beyond the abilities of the code currently, the same applies to a set of elements only being valid within certain elements (ul, ol can only contain only contain li elements) which doesn't happen at the moment. That and adding descriptions to all the tags/attributes (#323) I think should follow this. I made another issue to that effect #375

[dstorey]

Sounds good

[aeschli]

Thanks @Tyriar, very nice pull request!

[zbandhan]

And also HTML role attribute should auto-complete.

[Tyriar]

@zbandhan that was added in 98f9564 and successive commits.