Add `allowDuplicateProperties` parsing option, and make it the default.

While discouraged, the JSON spec does allow the same property to be
present more than once in an object. The built-in node.js JSON parser
allows them, but json-bignum previously did not. This option (default
true, changing the previous behavior) makes json-bignum behave the
same way as JSON in this respect.

For example, consider the following JSON string:

`{"hello": "world1", "hello": "world2"}`

With `allowDuplicateProperties == false`, parsing above will throw a
SyntaxError. When `true` (new default), it will return:

`{"hello": "world2"}`