Add new Props and new State keys [ Feature Request ]

[goxr3plus]

This is still missing in https://github.com/facebook/react-devtools but i don't see it neither here

For example on the right panel :

instead of only being able to edit the props and state we should be able to add new key/values to them :) Pretty useful feature actually .

Is it hard to implement ? :) ?

[bvaughn]

Duplicate of #300