Handle opening Chrome when Firefox is the currently active window and
vice versa. Also added support for Chrome's Incognito and Firefox's
Private Browsing mode.

- The keywords 'ff' and 'firechrome' will open Chrome if Firefox is
  the currently active window or Firefox if Chrome is the currently
  open keyword.

- The keyword 'iff' will open the currently active Chrome tab's URL in
  Firefox's Private Browsing mode. Conversely, if Firefox is the
  currently active window, it'll launch a new Chrome Incognito window.