Firefox Add-on; far easier than expected

It's early days yet, but this extension does work! :smile: 

It adds a button to the browser toolbar (using our `favicon.ico` atm) which, when clicked, loads h. into the page.

It does load on PDF's, but has issues yet to be understood/resolved. It also doesn't (yet!) avoid loading if the page already has h. embedded (ala http://webplatform.org/ ).

But it's a start! :tada: