[low priority refactoring] generalize executing JS during scraping

If doc sites use client-side JavaScript to produce or fetch some of their content, we need to launch a headless browser and execute it. Currently scraper subclasses do that individually using capybara, e.g.
https://github.com/freeCodeCamp/devdocs/blob/master/lib/docs/scrapers/angularjs.rb#L42-L45
but it might be better to extract a helper method or config option, a little more like this:
https://github.com/freeCodeCamp/devdocs/blob/master/lib/docs/scrapers/webpack.rb#L43-L46

cc @dangeranger