[boostbook] Supporting variadic template parameters

Hi,

I'd like to add support for variadic template parameters to BoostBook.
I've attached a patch showing what I've done so far - basically I'm
adding a variadic attribute to template-type-parameter and
template-nontype-parameter, for example:

  <method name="emplace">
    <template>
      <template-type-parameter name="Args" variadic="1">
      </template-type-parameter>
    </template>
    <parameter name="args">
      <paramtype>Args&amp;&amp;...</paramtype>
    </parameter>
    <type>std::pair&lt;iterator, bool&gt;</type>
  </method>

for:

  template<typename... Args>
  std::pair<iterator, bool> emplace(Args&&... args)

You can see the output on:

http://unordered.nfshost.com/dev/doc/html/boost/unordered_set.html

Does this seem like a good solution? Giving the attribute a "1" value
seems very awkward, but I couldn't think of anything better (I copied
it from the 'html-only' attribute). Maybe it would be better to add