Draft implementation of @require functionality as defined in LearnBoost/stylus#1239

- Add a Require node
- Add the node to lib/nodes/index.js exports
- Add a visitRequire method to lib/visitor/compiler.js
- Add new atrule to lib/lexer.js around here
- Add a new method like this to lib/parser.js
- Add a visitRequire method to Evaluator.prototype in lib/visitor/evaluator.js
- 1 (not too simple) test passing