- Refactor Context from just variables to an object holding
  both the variables and a reference to the engine.
# Preparing implementation of partials
- QA: Rename values -> variables
- Delegate parsing back to engine
- Change render() and transform() to accept either variables
 or a Context object, and then change Sections to reuse the context
- Move parsing back to engine
# Preparing partials, part 2
- Implement partials
  . Standardize file extension at ".mustache"
  . Remove file extension from transform() call
  . Add support to parser
- Implement partials
  . Create PartialNode class