Emoji Syntax Gravatars

*Fixes #56*

* Propose $simeonwillbanks$ syntax; $ as delimiter
* Add AvatarFilter test for template method
* Add GravatarFilter tests for parsing and various context options
* AvatarFilter defines interface
* GravatarFilter defines concrete implementation
  * GravatarFilter requires service for converting username to email

*Notes*
- Initial implementation for discussion
- Needs TomDoc