Move grammar parsing into new Grammar class

lib/propane/rule.rb

@@ -2,16 +2,37 @@ class Propane
 
   class Rule
 
-    attr_reader :name
-
+    # @return [Array<Token, RuleSet>]
+    #   Rule components.
     attr_reader :components
 
+    # @return [String]
+    #   User code associated with the rule.
     attr_reader :code
 
-    def initialize(name, components, code)
+    # @return [Integer]
+    #   Line number where the rule was defined in the input grammar.
+    attr_reader :line_number
+
+    # @return [String]
+    #   Rule name.
+    attr_reader :name
+
+    # Construct a Rule.
+    #
+    # @param name [String]
+    #   Rule name.
+    # @param components [Array<String>]
+    #   Rule components.
+    # @param code [String]
+    #   User code associated with the rule.
+    # @param line_number [Integer]
+    #   Line number where the rule was defined in the input grammar.
+    def initialize(name, components, code, line_number)
       @name = name
       @components = components
       @code = code
+      @line_number = line_number
     end
 
   end