Tag datastructure definitions in docs/spec/* for highlighting

Currently the language used for highlighting is auto-detected and not ideal.
A datastructure-definition lexer should be created to optimize highlighting.

Some specifications have links to other parts of the documentation; currently
these would get escaped and highlighted (and so these specifications have not
been tagged). The HighlightExtension needs extending so that sections of text
to not highlight (or escape) can be marked. Alternatively, the HTML formatter
supports adding links using a ctags file, but this would require formalizing
the datastructure-definition format enough to be able to identify names
properly.

i2p2www/pages/site/docs/spec/streaming.html

@@ -12,7 +12,7 @@ See the Streaming page for an overview of the Streaming Library.
 <p>{% trans -%}
 The format of a single packet in the streaming protocol is:
 {%- endtrans %}</p>
-<pre>
+{% highlight %}
 
 +----+----+----+----+----+----+----+----+
 | send Stream ID    | rcv Stream ID     |
@@ -29,7 +29,7 @@ The format of a single packet in the streaming protocol is:
 +----+----+----+----//
 
 
-</pre>
+{% endhighlight %}
 
 <table>
 <tr><th>{{ _('Field') }}<th>{{ _('Length') }}<th>{{ _('Contents') }}