Added getCurrentTokenTree to get a token's tree within a helper

I have a custom Helper where I wish to get the piece of the tree associated with my token in order to "reflect" the contents. Currently, I am limited to getting the template's full tree through the Template instance. The only way to do what I want was to re-parse the $source variable that is passed to the helper, which is OK, but felt very inefficient. I think this is a pretty simple solution to the problem at hand, but let me know if there is a better way, better naming, etc. Thanks!