[vim] Add full HTML5 support

Vim's default HTML support doesn't include full highlighting for HTML5
stuff, just the HTML4 compatible subset. That works fine, but this is
better.
[vim] Add zencoding support (for HTML)

Zencoding is an alternate shorthand for writing HTML. Something like

    div.important>p

would expand to

    <div class="important">
        <p></p>
    </div>

which could then be filled in with content as desired.