Allow user to specify custom token node fields

spec/test_token_user_fields.d

@@ -0,0 +1,24 @@
+import testparser;
+import std.stdio;
+
+int main()
+{
+    return 0;
+}
+
+unittest
+{
+    string input =
+        "# c1\n" ~
+        "#  c2\n" ~
+        "\n" ~
+        "first\n" ~
+        "\n   \n  \n" ~
+        "  # s1\n" ~
+        "   #   s2\n" ~
+        "second\n";
+    p_context_t * context;
+    context = p_context_new(input);
+    assert(p_parse(context) == P_SUCCESS);
+    Start * start = p_result(context);
+}