Switch from Chrome-specific API to browser-agnostic API

[rossjrw]

This PR switches from the chrome api to the browser API, with the webextensions-polyfill library acting as a polyfill for Chrome (which doesn't implement the WebExtensions API directly). This will enable the extension to work on all browsers except for Safari.

This PR also cleans up the JS a little bit.

This PR requires #8 but merges into master, so after #8 is merged, the base branch should be changed to master.

• Use browser-agnostic API
• Export a ZIP bundle (which is needed for testing on Firefox)
• Get it working

Later, we'll need to work out signing for Firefox addons, as unsigned extensions are blocked from being added to the browser (they can only be added to Developer Edition and even then only after toggling a config flag). There are a lot of hoops to jump through. In the meantime, the extension can be tested on Firefox Developer Edition by setting xpinstall.signatures.required to false in about:config.

[rossjrw]

Noting that at this point the extension loads fine in both Firefox and Edge without errors (although Edge complains about browser_specific_settings not being a valid key, which it is wrong about), and the options page works as expected, but no changes are visible on the wiki and there are no errors in the console. Did I get something wrong re: requesting permissions? It worked in Edge as of #8...

Update: I forgot to re-add document.append