Add grammar syntax to specify lexer mode for tokens and patterns

lib/propane/pattern.rb

@@ -26,6 +26,10 @@ class Propane
     #   Regex NFA for matching the pattern.
     attr_reader :nfa
 
+    # @return [String, nil]
+    #   Lexer mode for this pattern.
+    attr_reader :mode
+
     # Construct a Pattern.
     #
     # @param options [Hash]
@@ -40,12 +44,15 @@ class Propane
     #   Token to be returned by this pattern.
     # @option options [Integer, nil] :line_number
     #   Line number where the token was defined in the input grammar.
+    # @option options [String, nil] :mode
+    #   Lexer mode for this pattern.
     def initialize(options)
       @code = options[:code]
       @drop = options[:drop]
       @pattern = options[:pattern]
       @token = options[:token]
       @line_number = options[:line_number]
+      @mode = options[:mode]
       regex = Regex.new(@pattern)
       regex.nfa.end_state.accepts = self
       @nfa = regex.nfa