Code piece toggle and source download

By Nir Tzachar request, added 2 new useful features for CODE tag.

Expand/collapse toggle button for long code pieces (height ≥ 250px).
Download highlighted source code as text file. Enabled on <code lang="..."> tags and supports custom filename via filename="..." attribute.

Features implemented using jQuery library.

An example (actual JS code, look at right upper corner):

$(document).ready(function(){
$('div.code').each(function(){
var extra = '';
var fname = $(this).attr('filename');
if(!fname) fname = 'code.txt';
if(fname)
extra += '<a onclick="get_code_source(this.parentNode.parentNode)">'+
'<img src="images/download.gif" alt="download" title="download '+fname+'"/></a>';
if($(this).children('.hl-main').height() > 200)
extra += '<a onclick="toggle_code(this.parentNode.parentNode)">'+
'<img align="top" src="images/toggle.gif" alt="toggle" title="toggle"/></a>';
if(extra)
$(this).prepend('<div class="small float-right no-print">'+extra+'</div>');
});
});