@storybook/addon-knobs: add button() knob

[gustavohenke]

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

This knob was introduced by storybookjs/storybook#2004 in v3.2.13 of Storybook.

[typescript-bot]

@gustavohenke Thank you for submitting this PR!

🔔 @joscha @martynaskadisa - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[joscha]

should we bump the types version to the minor that contains the new button element?

[typescript-bot]

A definition author has approved this PR ⭐️. A maintainer will merge this PR shortly. If it shouldn't be merged yet, please leave a comment saying so and we'll wait. Thank you for your contribution to DefinitelyTyped!

[gustavohenke]

should we bump the types version to the minor that contains the new button element?

I didn't read the rest of the changelog from v3.2.1 to v3.2.13 to see if other meaningful changes were introduced though!

[joscha]

I didn't read the rest of the changelog from v3.2.1 to v3.2.13 to see if other meaningful changes were introduced though!

well this is only the knobs portion, so it should be okay.