Support else in if then conditions

source/lexer.ll

@@ -37,6 +37,9 @@ if                      {
 then                    {
                             return yy::parser::make_THEN(this->location);
                         }
+else                    {
+                            return yy::parser::make_ELSE(this->location);
+                        }
 while                   {
                             return yy::parser::make_WHILE(this->location);
                         }