Moved Firefox to web extensions

[twolfson]

As reported in #47, Firefox 57 no longer supports the old extension format (i.e. .xul and friends). As a result, we need to move to web extensions which are 1:1 with Chrome extensions. In this PR:

• Copy/pasted/tweaked Chrome extension into Firefox web extension
• Updated compile/deploy script
• Fixes Support WebExtensions of Firefox extension #47

Notes:

This was relatively straightforward; mostly replacing chrome with browser makes it work. One sticking point was the deprecated tabs.onSelectionChanged API but that was easily resolved with its counterpart tabs.onActivated:

https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/tabs/onSelectionChanged

Relevant links:

• https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Porting_a_Google_Chrome_extension
• https://developer.mozilla.org/en-US/Add-ons/WebExtensions#Firefox_workflow

[twolfson]

Proof of functionality can be found here -- I submitted it to addons.mozilla.org via the scripts in this PR:

https://addons.mozilla.org/en-US/firefox/addon/livereload-web-extension/

[GCorbel]

It works with Firefox quantum. Thanks.