Add Firefox support

[KarimPwnz]

I've added Firefox support with the following changes:

• Added a /s flag equivalent in the wrapper detection of content_script.js (reason: https://bugzilla.mozilla.org/show_bug.cgi?id=1361856)
• Changed the way popup.js gets data from background.js; it now uses runtime.getBackgroundPage, which is less buggy in Firefox and more conventional in general
• Renamed chrome/ to src/
• Added a signature to manifest.json—this causes a "warning" in Chrome, but it is insignificant
• Defined the HTML encoding of the HTML documents (this removes an annoying warning)
• Added a signed XPI (postmessage_tracker-1.0.1-fx.xpi) from https://addons.mozilla.org/
• Updated the version to 1.0.1

This porting works because Firefox supports the chrome namespace.

[KarimPwnz]

If you'd like to maintain the XPI yourself, go to https://addons.mozilla.org/, upload the extension, and change the information in manifest.json.

[KarimPwnz]

Do note that the signing is necessary to install the extension. There is no "developer mode" in standard Firefox.

[vzamanillo]

I can confirm that this works on FF 76.0 (64-bit) Linux, good job, waiting for the merge.