Give Shipping Methods Control Over Their Cart Full Label

The idea is to give shipping methods more control over their cart full label by adding a filter to the cart shipping methods template.

Actually I was somewhat surprised that the code to generate the full labels is entirely within the template file; it seems perhaps a little involved to live within a template file?  I played around with moving that logic into the new WC_Shipping_Rate class as a get_full_label() method, to reduce the complexity within the template, but then I wasn't sure whether there might be good reason that code should be where it currently is.  

Either way having a means for plugins to control the final label display is a important, especially with 2.0 now automatically adding the '(Free)' text.