Add support for `Symbol` index signatures in the state

[ahtcx]

Currently redux-devtools seems to only display enumerable properties on objects - it would be nice for it to display it more like Chrome/FF dev-tools do.

[zalmoxisus]

That should be implemented in react-json-tree. If someone wants to contribute it there, that would be very helpful. See the discussion in alexkuz/react-json-tree#69 on how that was implemented for Maps.

[zalmoxisus]

I looked into it, apart from react-json-tree (which would bring it to inspector and log and monitor), it should be also implemented in jsan to be able to serialize from client part to the extension, in javascript-stringify to show as "raw" object (in inspector subtab), and in map2tree for chart monitor.

In case someone wants to work on this, here's an example to use:

var sKey = Symbol('SPECIAL_KEY');
var obj = {};
obj[sKey] ='this is getting lost now';