Allow user-defined context fields

spec/test_user_context_fields.d

@@ -0,0 +1,19 @@
+import testparser;
+import std.stdio;
+import testutils;
+
+int main()
+{
+    return 0;
+}
+
+unittest
+{
+    string input = "aaa\n\n\na\n    # comment 1\na  a    aa\n\naa\n#    comment 2\na\n";
+    p_context_t context;
+    p_context_init(&context, input);
+    assert(p_parse(&context) == P_SUCCESS);
+
+    stderr.writeln("comments: ", context.comments);
+    stderr.writeln("acount: ", context.acount);
+}