Add support for @value attribute

[gkellogg]

From the RDFa+HTML errata:

HTML’s <data> element and its @value attribute are currently not defined for an RDFa processor, although it could be used to generate literal values. It could also be useful to define @value for RDFa data processing of other elements where it is allowed, namely:

• <li> elements inside <ol> lists. There are cases where it's semantically relevant to identify the position of an object in a series. HTML allows it by giving an explicit value to list items where order matters (though this would force authors to use another element, e.g. <span>, to mark up the real content of the list item, but it seems reasonable).
• <input> (especially when in read-only mode, these are cases where @value matters).