Allow for distinct JSONLogic instances

[briandamaged]

This change exposes the JSONLogic factory function so that multiple independent instances of JSONLogic can be instantiated at once. For example:

// The module itself still acts like a JSONLogic instance.
const defaultInstance = require('json-logic-js');
defaultInstance.apply({"<": [2, 3]});

// However, now you can create additional instances of
// JSONLogic as well.  Each instance can have its own
// custom operators.
const myInstance = new defaultInstance.JSONLogic();

myInstance.add_operation("join", function(ary, glue) {
  return ary.join(glue);
});

myInstance.apply({"join": [ ["foo", "bar"], ","] });

[briandamaged]

Alright -- here's the re-implementation of #70 . This does the same thing, but it eliminates the re-indentation.

[briandamaged]

@jwadhams : ping

[josephdpurcell]

I like this idea! I'm a bit new to JSON Logic, but in the back of my mind I was thinking about how when you add operations it would be added globally. On principle, it would be nice to have the ability to have multiple instances of JSON Logic.