Add HTML5 'required' attribute to form widgets.

[ghost]

Hello @artyom-beilis,

In current version, CppCMS does support 'disabled' attribute on form widgets. But, if a developer would like to add 'required' attribute to form widgets in templates, he/she is required to write that part manually each time.

This commit provides code for 'required' attribute.

1. Changes in the commit are following 'disabled' attribute's implementation.
2. non_empty() function automatically calls required(true).
3. Call to at_least with positive number automatically sets required attribute.
4. Call to limits with positive min argument automatically sets required attribute.

To not break backward compatibility, required flag is not checked if the widget have non_empty(). But this may cause confusion for new developers, so what's your opinion? Should CppCMS check even if non_empty() function is present?

Thanks.