Add a feature for taking screenshot of a particular window in desktop.

[SakthiMahendran]

This feature will be very useful and has many use cases.

Currently I believe there is no cross platform way to do this in go.

What we can do currently is use the underlying OS API to get this done.

But if you add feature to do this in a cross platform way by utilising all the OS API it will be great help :).

[vcaesar]

I will add it in the future.