links open in external window via subclass of Redcarpet renderer

@kevsherman @gordonwilkinson This class shouldn't be in the helper. Please put it in as an initializer. Also, this code is not self explanatory.. I assume you found it somewhere and couldn't just add it to the options hash above; 

Please include a link to the resource that lead you to put in this code, so future devs can figure out how you arrived at it.