Refactor collectstatic by extracting parts of copy_file into other methods.
Rename BaseFilter methods for consistency and start work on additional mixins.
Add SingleInputMixin
Add MultiInput mixin which can accept a glob pattern.
All filters will now use a set input_path or output_path if specified.
Fix some Python 3 compatibility issues
Refactor collectstatic to allow for post processing. fixes #1
Version bump