Added: validate methods (no checks needed) to variable, command, comment, literal and log nodes.
Added: validation for 'Usage' block; other blocks do not need any validation.
Removed: 'operator' node interpreter files.
Added: validation method for special-word (no validation needed)
Added: a root node to the parser output. Fixed: position#toString.
Refactor: break out interpreter tests to multiple files
Added: Node constructors for the different node types.
Convert raw parse tree to JS object tree.
Added: a ValidationError type
Added: Interpreter validate, eval and run methods. [no-tests]
Added: root node validation on the interpreter.