Add plugin hooks to customize page meta data

This change would allow any developer the ability to easily add custom meta data to their pages. This pull request implements the following two plugin hooks:

* `before_read_file_meta` - Executed in the `read_file_meta` method directly after the initialization of the `$headers` array. The callback function will be passed the `$headers` array by reference for manipulation.
* `get_page_data` - Executed in the `get_pages` method for each page after the initialization of the `$pages` array. The callback function will be passed the `$data` array by reference as well as the `$page_meta` array for including extra meta data.