Nodejs port of python defaultdict
var DefaultDict = require('defaultdict')
// create dict
var dict = new DefaultDict(0);
// works like normal dict
dict.set('foo', 'bar');
dict.get('foo');
//=> 'bar'
// get undefined keys
dict.get('qux');
//=> 0
// add a value to a key (must support + operator)
dict.add('qux', 5);
dict.get('qux')
//=> 5