Add tests for different interpolation types
Add tests for different interpolation types

Instance var (#@foo) and global (#$foo) styles aren't interpolated. Add
tests to expose this.

sideci-sample/sideci-sample-haml,dg1dd1ngs/haml,macressler/haml,dg1dd1ngs/haml,stanhu/haml,haml/haml,keithpitty/haml,nazar-pc/haml,keithpitty/haml,nazar-pc/haml,stanhu/haml,tangposmarvin/haml,haml/haml,macressler/haml,haml/haml,haml/haml,tangposmarvin/haml