Improvement: Use declarativeContent to do domain matching

Older versions of the `pageaction_by_url` sample used [simple `.indexOf` matching](https://github.com/neopunisher/Chrome-Developer-Samples/blob/master/examples/api/pageAction/pageaction_by_url/background.js) to see whether a domain/URL matches or not. This plugin basically does the same.

Newer versions of the `pageaction_by_url` sample however [seem to use](https://chromium.googlesource.com/chromium/src/+/master/chrome/common/extensions/docs/examples/api/pageAction/pageaction_by_url/background.js) something named [`declarativeContent`](https://developer.chrome.com/extensions/declarativeContent) for its URL matching.