Update extend.Filter API
- Add more type and `pre` is renamed to `beforePostRender`, `post` is renamed to `afterPostRender`
- Add Filter.apply
- Add Filter.unregister
- Add filter priority
Use native url library in external_link filter
Add new_post_path filter
Filter.apply: return the last value in sync mode
Filter.unregister: Unregister all plugins if `fn` is not defined
Add post_permalink filter
Rename filter types
Filter.apply: Add `context` argument
Merge branch 'feature/enhance-filter' into dev