Added support for the crossmatch plugin to load external task configs.

This is in regards to http://flexget.com/ticket/2254.
This is my first attempt at hacking flexget code, so I am completely open to input. :)

Notes:
- I pulled the method 'load_plugin_config' out of the Include plugin and left it in the plugin module. It felt like this should be pulled out some place instead of duplicating the yaml.load and validation logic but I was uncertain where to put this. Any thoughts?
- I added a new property to the crosscheck plugin named 'include'. This is due to perceived schema limitations. It also looked cleaner to separate out the loading of a file from actual configuration you would fine in the 'from' section.