Post Processor Plugin

I was reading an issue from a several months ago that was requesting the `_pages` directory get special treatment. The request was rightly rejected from being added to the core, but I decided to try to write a plugin to address that issue. 

I found that it wasn't possible because moving files from `_pages` to `..` would have to happen after the site has been built and is sitting in the destination folder, but Generators run before before rendering or writing. 

I realized that a post processor plugin type could be useful in a lot of situations, including the example `_pages` situation, so I wrote it.