Adding custom bleed options for left and right pages

Alright, this is my first attempt at adding something! I need to be able to define custom bleed options for left and right pages, so I added that option. The way this code currently works is that a main bleed still needs to be set at the root "@page" level before the right/left bleeds will even be parsed, which can probably be improved in future.

This is not the DRYest code in the world, but it works! I tried to follow the same formatting/structure/naming conventions that you guys have been using. And the tests passed as far as I could tell. Let me know if you want me to make any changes.