Update components to support both href and onClick

[chrisdavies]

Fixes #553 and Fixes #558

[snide]

This works as advertised and the functionality seems fine. If this is the pattern we're going with can we...

1. Update the docs to show a live example.
2. Can be a seperate PR, but we should update all the button components to do the same, since right now they operate in the reverse and could probably accept both as well.

[snide]

Also don't forget a changelog item.

[chrisdavies]

Ah. I had failed to read the contributing guide for eui. Sorry about that!

[chrisdavies]

Hm. Button does this slightly differently. Instead of checking if href is undefined, it checks if it's falsy. I don't know which is preferable. Thoughts?

[chrisdavies]

FYI, hold off on reviewing until my next comment. I'm going to go ahead and make the button changes in this PR, since it makes some cleanup easier.

[chrisdavies]

I forgot to give the all-clear. This is once again ready for review!

[snide]

This looks ok to me, tested it on all our button types. Thank you for adding it to the cards as well. Only rec might be to add some notes in the prop docs, but feel free to merge

[chrisdavies]

@snide Thanks! I just took a look at the props. They don't seem to conflict with this change, but agree, in general, they're pretty sparse. I think I'll check this in, though.