doc: add section listing template functions/fields provided by plugins

This PR adds a section at the end of the `reference/pathformat.html` page to list functions/fields provided by plugins.  

Plugins related to path-formats are already listed [here](http://beets.readthedocs.org/en/v1.3.5/plugins/index.html#path-formats) but :  

- list is (understandably) incomplete as some plugins belongs to many categories cf `missing` plugin
- it still requires reader to click each plugin link to go grab the functions/fields