add loading state to alerts

[nnchang]

I ran into a situation where I wanted to make an alert that had a clear load state (specifically the one supplied by <Button>). Because <Alert> doesn't give allow much control over the buttons, I wound up implementing that solution using a dialog and styling it to look like an alert. I think it would be nice for <Alert> to have this functionality.

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

Add a loading state to <Alert>.

Reviewers should focus on:

I'm not familiar with the testing library, so I haven't yet added tests. I can dig into that if maintainers find this to be an acceptable suggestion.

Screenshot

[nnchang]

side note: as a new contributor, I was surprised that the yarn dev command as suggested in the CONTRIBUTING.md took me straight to the table app and didn't seem to have the core package building. Looking at package.json, I think this is expected behavior. I'm happy to make a PR to change the docs if appropriate.

[adidahiya]

good point on yarn dev from root. I made a change in 842b2bb but forgot to reverse the negation in the glob. I'll fix that.

it looks like you may have enabled CircleCI for your fork, which is not encouraged -- we want your PR to build on our org's Circle instance to report status checks correctly.

as for the new prop itself... I am generally very conservative about what we add to Alert but in this case I think it's an elegant addition and I support it 👍

[nnchang]

cool! I will close this PR and try to reopen such that circle will run correctly.