More Powerful Filter API

Currently Filter API only has 2 types: **pre** and **post**. I decided to reference the [Filter](http://codex.wordpress.org/Plugin_API/Filter_Reference) of WordPress and make the Filter API more powerful.

The old filter types are renamed, which is easier to understand. However, you can still use the old filter types.

- **pre** → **before_post_render**
- **post** → **after_post_render**