Add support checking alt attribute of area[href] and input[type=image]

Area and input elements require alt attribute in some condition likewise alt attribute of img element.

This rule check whether area and input element have non empty alt attribute.
http://www.w3.org/TR/html5/embedded-content-0.html#attr-area-alt
http://www.w3.org/TR/html5/forms.html#attr-input-alt