Templating: allow marking a node so that its contents aren't interpreted

[Tarmil]

Add a new attribute ws-preserve such that children aren't interpreted for template holes and ws- attributes and nodes. eg:

<div>
    ${ThisIsAHole}
    ${ThisToo}
    <div ws-preserve>${ThisIsNotAHole}</div>
</div>

filled with:

MyTemplate().ThisIsAHole("foo").Doc()

gives:

<div>
    foo

    <div>${ThisIsNotAHole}</div>
</div>