Add same attribute tuple support for `for` attribute (for matching ids)

[kevinastone]

Can you allow the same tuple for a for attribute as supported by id (joined as '_') so we can do something like this:

%label{'for': ('id', '{{ control }}')}
    = control|title
%input{'id': ('id', '{{ control }}'), 'value': 'Value'}

This would be especially useful when having lists of controls so we could vary the prefix.