Add detection of GrammarKit-generated files

GrammarKit is a plugin by JetBrains for creating custom language plugins
for JetBrains IDEs (such as IntelliJ, RubyMine, CLion and more). It
defines a BNF parser language which can be used to generate a parser in
Java, and it also integrates JFLex for generating a lexer in Java.

Both of these generated Java files can be recognised by a comment on the
first line of the file, and so classifying them as generated is trivial.

lib/linguist/generated.rb

@@ -72,7 +72,9 @@ module Linguist
       vcr_cassette? ||
       generated_module? ||
       generated_unity3d_meta? ||
-      generated_racc?
+      generated_racc? ||
+      generated_jflex? ||
+      generated_grammarkit?
     end
 
     # Internal: Is the blob an Xcode file?
@@ -373,5 +375,32 @@ module Linguist
       return false unless lines.count > 2
       return lines[2].start_with?("# This file is automatically generated by Racc")
     end
+
+    # Internal: Is this a JFlex-generated file?
+    #
+    # A JFlex-generated file contains:
+    # /* The following code was generated by JFlex x.y.z on d/at/e ti:me */
+    # on the first line.
+    #
+    # Return true or false
+    def generated_jflex?
+      return false unless extname == '.java'
+      return false unless lines.count > 1
+      return lines[0].start_with?("/* The following code was generated by JFlex ")
+    end
+
+    # Internal: Is this a GrammarKit-generated file?
+    #
+    # A GrammarKit-generated file typically contain:
+    # // This is a generated file. Not intended for manual editing.
+    # on the first line. This is not always the case, as it's possible to
+    # customize the class header.
+    #
+    # Return true or false
+    def generated_grammarkit?
+      return false unless extname == '.java'
+      return false unless lines.count > 1
+      return lines[0].start_with?("// This is a generated file. Not intended for manual editing.")
+    end
   end
 end