Break bfsTransform into bfsTransform and bfsTraverse

[gaurav21r]

https://github.com/rcorp/jequel/blob/master/src/bfs-transform.coffee is pretty big and contains two different logical components.

We should break this down into bfsTraverse and bfsTransform. bfsTransform would be the same as before. bfsTraverse needs to have a simple API, where we would execute a function for every node traversed via Breadth First Search.

API and examples in my next comment

[gaurav21r]

Signature of bfsTraverse according to me should be

bfsTraverse (objectToTraverse: {}, functionToCall: Function)
functionToCall (node: Object, parent: Object)

node, path = {
  path: <path of current node as a JSON Pointer>,
  value: <value of current node>
}

The Algorithm should be the one given in Wikipedia, https://en.wikipedia.org/wiki/Breadth-first_search#Pseudocode

@dpchn @prakash1517