Added filters, and more

- Added custom filters for Liquid
- Refactored usage of 'site' throughout ink plugins
  Since it was just being used for configs, using an instance variable
  on @plugins instead of passing site all around to every function.
- Fixed issue where conditionals could cause filters not to run
simplified var helper a bit
Added Liquid filter support to inlcude, render, and yield.
Also added a new line comment tag for easy inline comments with minimal markup.