Add hooks to allow content (web pages) to capture a screenshot for reporting site issues

[cvan]

Just like here in the Firefox extension, can we add a hook that can let a web page communicate with the extension to take a screenshot (either by postMessage, a DOM API added by a WebExtension-compatible content script, etc.)?

The screenshot bits are the most important for us, when we direct people to a new WebCompat issue form from a link in content on a WebVR page.

[cvan]

Related bits from Safari extension: https://github.com/webcompat/webcompat-reporter-extensions/blob/d0fb595/safari/webcompat-reporter.safariextension/sendScreenshot.js#L8

We'd need more than that (not just whitelisted for the webcompat.com origin), but that's the general idea of what I was thinking.