Added more useful shortcuts for switching between browsers

- krome does the reverse of what the original extension did - it opens
the currently open FF tab in Chrome
- incognito (also bound to Shift+Command+M) opens the current tab in
Chrome in Incognito mode
Updated README and screenshot to reflect the additional keywords
Fixing minor typo

Opne -> Open
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.
Updated subtext to indicate that ff and firechrome keywords are bidirectional.
Merge pull request #2 from deepakg/master

Making Firechrome "bidrectional"