File identifier feature.

Allows you to add, and test for the presence of a magic 4-char
string in a FlatBuffer.

Tested: on OS X.

Change-Id: I090692a9e4fb53bed3543279a28563e67132cba0

include/flatbuffers/idl.h

@@ -284,6 +284,8 @@ class Parser {
 
   FlatBufferBuilder builder_;  // any data contained in the file
   StructDef *root_struct_def;
+  std::string file_identifier_;
+  std::string file_extension_;
 
  private:
   const char *source_, *cursor_;