Add JUMP_TO_STATE action for slider-like monitors

[gaearon]

I challenged you to implement slider-like custom monitors in #3, but in fact I forget something essential to make it easy.

The built-in LogMonitor lets you “toggle” individual actions, disabling them from history. This is not what slider interface is like though: a slider just moves back and forward in time between the computed state. It does not really alter the history: it only alters the current state we are looking at.

This PR adds another devtools action: JUMP_TO_STATE. By default, the currentStateIndex will advance any time an action is performed, but your custom monitor can also call this.props.jumpToState({ index: 0 }) to jump to the first state, or something in the middle (make sure to to put there values larger than this.props.computedStates.length - 1.

The default LogMonitor does not use this.props.jumpToState at all, but your custom monitor may use it, as well as this.props.currentStateIndex, to implement a slider-like UI.