RFC7159 mode

This adds a new "mode" to JSON which parse/generates JSON according to RFC7159 (the most up-to-date JSON spec), a fix for #206. The main difference between RFC4627 (the default in the JSON gem) and RFC7159 is that the latter RFC accepts JSON values as a valid JSON text.

You can use the new mode by passing `standards_mode: true` to `parse` (in a similar way to how `quirks_mode` is activated).