[Feature] Support custom runner script to exercise the JavaScript code

[aminya]

According to readme:

UnCSS only runs the Javascript that is run on page load. It does not (and cannot) handle Javascript that runs on user interactions like button clicks. You must use the ignore option to preserve classes that are added by Javascript on user interaction.

If we can provide an option to allow the user to specify a certain script that exercises the JavaScript code, we can remove this limitation. For example, they can write a script to click that button.

Here is a tool by @toshi1127 which uses puppeteer's coverage feature to remove unused css. If we can integrate these two projects together, we can get very good results!
https://github.com/toshi1127/css-optimization