That commit incorrectly caused a bunch (if not all) custom stylesheets
to be inserted for the wrong domains.

Add a condition to ensure that the domain of the `tabId` used matches
that of the stylesheet.

When opening tabs in the background, for example when clicking links,
custom CSS wouldn't get properly loaded in those tabs.

It turns out that the CSS was actually getting loaded into the current
tab, which I hadn't realised.

Thanks to Niklas Gollenstede's answer here for clueing me into this:
https://discourse.mozilla.org/t/inject-css-with-webextension/16877/3

To fix the problem, we specify the `tabId` in which to load the CSS. We
get the `tabId` from the `webNavigation.onCompleted` event, which tells
us the page is basically ready.

Forgot this when I wrote the instructions. I usually like to prefix
shell commands with a prompt character.

Includes instructions for Chrome and Firefox. Firefox is a little more
involved. For now not bothering to sign the extension.

Also update `manifest.json` to give the extension an add-on ID. This is
required as it must be used as the directory name in the Firefox
`extensions/` profile folder.

Thanks to these resources for explaining how to install an unpacked
extension in Firefox:
- https://stackoverflow.com/questions/27155766/how-to-install-unpacked-extension-in-firefox-chrome/27158189#27158189
- https://developer.mozilla.org/en-US/docs/Archive/Add-ons/Setting_up_extension_development_environment#Firefox_extension_proxy_file
- https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Alternative_distribution_options/Sideloading_add-ons
- https://stackoverflow.com/questions/37728865/install-webextensions-on-firefox-from-the-command-line/37739112#37739112

Add an example to show how the extension works.

Write a description of the web extension. Start on installation
instructions.

Include the subdomain when matching filenames. This is less general, but
allows us to match more precisely. I needed to modify CSS on a
`subdomain.github.io` blog, and of course we don't want to modify _all_
`github.io` styles.

Ignore contents of the `/css/` directory, but don't ignore `.keep`
files, to allow the directory to remain tracked in the repo.