fix(button): Support anchor and button HTML attributes (#679)

[ghost]

I've chosen to extend React.AnchorHTMLAttributes<T> and React.ButtonHTMLAttributes<T> since these are the possible element types that can be rendered. However looking through the other components you seem to use HTMLProps<T> a lot as well. There doesn't seem to be any clear reason why this is used in some places and not in others.

I'm happy to change this to use HTMLProps<T> instead if that is the pattern you want to follow. It would make button consistent with icon-button.

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[codecov-io]

Codecov Report

Merging #680 into rc0.10.0 will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           rc0.10.0     #680   +/-   ##
=========================================
  Coverage     94.88%   94.88%           
=========================================
  Files            68       68           
  Lines          2852     2852           
  Branches        432      432           
=========================================
  Hits           2706     2706           
  Misses           50       50           
  Partials         96       96

Impacted Files	Coverage Δ
packages/button/index.tsx	96.66% <ø> (ø)	⬆️

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[ghost]

I signed it!

[moog16]

@ben-mckernan you're right - this should at the very least been extending from HTMLProps. Not sure how this one snuck through the PR process without it. This looks good to me, as you're right the button can be an anchor or button element.

Thanks for fixing.

[moog16]

CLA was not passing, but signed in #680 (comment)