Missing support for ES2015 Map and Set

[lmammino]

The code doesn't seem to work when the underlying object (or an object along the path) is of type Set or Map.

We could support both by following this approach:

• with Set, convert it into an array and access it as it is an array (some logic and memory considerations need to be made here)
• with Map, you can simply use the get and has fields to access the data.

Code to replicate the issue:

var objectPath = require("object-path");

var s = new Set(['a','b','c']);
console.log(objectPath.get(s, '0')); // undefined
console.log((Array.from(s))[0]); // 'a'

var m = new Map();
m.set('a','b');
console.log(objectPath.get(m, 'a')); // undefined
console.log(m.get('a')); // 'b'