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);
  '';
}
```

pkgs/applications/networking/browsers/tor-browser-bundle-bin/default.nix

@@ -37,6 +37,9 @@
 
 # Pluggable transport dependencies
 , python27
+
+# Extra preferences
+, extraPrefs ? ""
 }:
 
 with stdenv.lib;
@@ -197,6 +200,10 @@ stdenv.mkDerivation rec {
     // Defaults to creating $TBB_HOME/TorBrowser/Data/Tor/{socks,control}.socket
     lockPref("extensions.torlauncher.control_port_use_ipc", true);
     lockPref("extensions.torlauncher.socks_port_use_ipc", true);
+
+    ${optionalString (extraPrefs != "") ''
+      ${extraPrefs}
+    ''}
     EOF
 
     # Hard-code path to TBB fonts; see also FONTCONFIG_FILE in