Custom Rotten Tomatoes RSS Feed URLS

Added custom url support for Rotten Tomatoes RSS Feeds. I wanted the complete DVD list as the RSS instead of in theaters for instance and there was no way to do it except by changing the hard coded url in main.py. This code mimics the imdb format of allowing custom urls for the rotten tomatoes RSS feed. A list of RSS feeds can be found at

http://www.rottentomatoes.com/help_desk/syndication_rss.php

NOTE: This is only my third time ever initiating a pull request on a collaborative project on github so I apologize in advance if I did something wrong. Also, I made it more python-y per mano3ms recommendation.