Making Firechrome "bidrectional"

Hi Enrico, I took your feedback from the last time and incorporated it into the workflow. Please let me know what you think.

Summary of changes:

1. ff/firechrome keywords are now context sensitive. If Chrome is the currently active window, it'll launch Firefox and if Firefox is the currently active window, it'll launch Firefox.

2. I changed the incognito keyword to 'iff'. It does the same thing as ff but it opens the 'other' browser in incognito mode. i.e. if Firefox is the currently active window and you type iff it'll open Chrome in Incognito mode but if Chrome is the currently active window, it'll launch Firefox is Private Browsing mode. I think that stays true to the core philosophy of the workflow of allowing users to switch between browsers quickly.

3. I've dropped the earlier keyboard shortcut for the incognito keyword.

4. Updated the documentation and screenshot.

Thanks,
Deepak