WIP: Firefox addon + build process

**WARNING:** known sloppiness ahead. :smile_cat: 

To build:
```
./bin/hypothesis extension production.ini firefox http://localhost:5000 resource://firefox-at-hypothes-dot-is/hypothesis/data
```

To test:
1. Download the Firefox Add-on
2. ...install it...etc...
3. in `./build/firefox` run:
```
cfx run --binary-args="-url http://w3.org/DesignIssues"
```
(or whatever other URL you like to test with)

Firefox should open with the extension sitting in the top right of the toolbar.

### (Some of the) Known Issues:
* loads irregularly when browsing cached pages (back / forward, 304 Not Modified, etc)
** likely some other Firefox event that needs to be caught somewhere
* fonts are broken
** relative URLs in the CSS aren't loading the fonts (despite their being there)
** making them absolute `resource://` URLs in the CSS file does the trick (but would take some ugly build stuff)
* **all** of the build process for this thing *can* and *should* be heavily refactored
** it's mostly copy / pasting to jump that hurdle and focus on the extension stuff
** fully planning to improve it

Enjoy! :tada: 
parameterize this over the webassets base dir and it'll work for both builds