Add unit tests for core functions

[Emperor42]

There are no automated tests, so regressions are likely. Use Jest (or Mocha+Chai) to test:

createTagElement creates the right DOM node and dataset attribute.
renderTags populates the feed correctly and respects tagSpacingPx.
handleVisibilityChange correctly highlights active tags and builds the keyword pool.
fetchAndRenderAd falls back when no provider matches.
Add coverage badges to the README.