Fix issue 246

[kubo]

Close #246

This pull request consists of three commits.

1. The first commit adds the following method to Parser and use it in the parse_expression method with two arguments.

   std::shared_ptr<ExpressionNode> parse_expression(Template& tmpl)

   The method with one argument returns when unexpected token is found.

2. The second fixes function(num1 + num2, num3) seems to run as if it is function(num1, num2 + num3) #246. parse_expression with one argument is called recursively for each argument.

3. The third is optional. If you don't prefer it, I'll revert it. It calls parse_expression with one argument after a left parenthesis instead of counting the nested level by current_paren_level.

[pantor]

Thanks for the fix and for your patience!