templated attributes

This release adds support for Mustache-like template strings (previously supported only in text nodes) in t- prefixed attributes. This means that instead of having to write a JavaScript expression:

<a t-href="'#' + id">link</a>

you can do this, which is much more readable:

<a t-href="#{{ id }}">link</a>

(The contents of the {{ }} placeholders can be any valid JavaScript expression!)