This is a manual test for now.

* Compare element to undefined directly.
* Improve the injected landmarks test page by adding nesting.

* This is for Firefox users, to test the MutationObserver stuff.
* Use Mozilla addons-linter to check extensions before they are uploaded
  (currently Firefox only).

* Remove console output when disabling extension on certain URLs.
* Remove the "loading" ("...") indicator from the badge (it should be
  robust enough not to need this, and it should not expose such internal
  details to the end-user).
* Only say how long it took to find landmarks when they are (re-)found
  as a result of a MutationObserver.
* Make LandmarksFinder.filter() cleaner using map().

This contributes to #84.

* Roll back to deepmerge 1.5.x due to change in behaviour
  TehShrike/deepmerge#83
* Add command definition for main-landmark.
* Add translation text for main-landmark.
* Simplify command handling in background script.
* Make error throwing consistent.

All seems well except addons-linter is complaining about PNGs, though
they have always shown up fine...

Thanks TehShrike/deepmerge#84 :-).

* Remove unused this.currentLandmarkElement() from LandmarksFinder.
* Track which element is the <main>/role="main" element.
* Add this.selectMainElement() to LandmarksFinder.
* Handle the 'main-landmark' message in content.management.js.
* Add info on skipping to the main landmark to the README.

This completes the work started in
f1331fa and closes #106.

* Tidy up mutation observer setup stuff.
* Remove a logic error (spurious `return false`) from mutation observer
  mutation handler code.
* Make comments and spacing consistent.

Fixes #107.