Change how files are imported in unit tests
Refactor normalizer

Flatten nested rules, add `level` properties to nodes
Refactor compiler
Rename doubled to doubleColon
Make keyframes prefix a node property
Make property priority a node property
Rename keyframeList to ruleList
Inline Compilier.prototype.visitNode
Restructure compiler files

Move files under compiler/node/ to compiler/
Restructure parser & importer files

Move files under parser/node/ to parser/
Move files under importer/node/ to importer/
Restructure normalizer files
Restructure files
Move mediaType's modifier to node property
Refactor normalizer

Child node levels are calculated after being visited
Add more unit tests
Directly return JSON in grammar
Remove Node() constructor
Rename Err to RooleError
Generate inter-rule comments

First comment is always generated.
Comments before CSS rules are generated
Refactor grammar