Implemented all ASS tags in the parser.

- Simple tag parser functions are now generated via a factory, leading to lesser boilerplate.
- Renamed libjass.tags.Blur to libjass.tags.GaussianBlur. libjass.tags.Blur now refers to the {\be} tag.
- Renamed libjass.tags.Pos to libjass.tags.Position
- Loosened the requirements of color and alpha tags based on a real-world script.