tor-browser-bundle-bin: support injecting additional prefs

The `extraPrefs` parameter is injected verbatim into the mozilla.cfg
file.

Note that the syntax is a superset of the usual prefs.js syntax.  The
following procedures are of particular interest:
pref() to set a preference as if it had been toggled in about:config
defaultPref() to set the *default* value of a preference
lockPref() to set a preference & prevent further modification
clearPref() to reset a preference to its default state

Example:
```nix
tor-browser-bundle-bin.override {
  extraPrefs = ''
    // Increase default security level
    pref("extensions.torbutton.security_slider", 2);
  '';
}
```