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

[takenspc]

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

[yaniswang]

Two point:

1. Please use: var mapAttrs = parser.getMapAttrs(event.attrs) https://github.com/yaniswang/HTMLHint/blob/master/src/rules/jshint.js#L13
2. Include img tag is better? Discard the img-alt-require rule.

[takenspc]

Thanks for your review!

• Changed to use getMapAttrs.
• Add support alt of img
• Fixed some typos and coding style nits

[takenspc]

Thank you for your merging!