Added post_filters plugin
Added post_filters plugin

Plugins that extend PostFilter will get three
additional methods: pre_render, post_render and
post_write. These will be called when every page
or post is processed and allows for the plugin to
modify the post/page object directly to modify
the output beyond what can be done with just
markup.

albrow/blog-old,albrow/blog-old,albrow/blog-old