New attribute to flowchart.js

Added "arrow-end" option to flowchart attributes. Change according raphaeljs documentation [[1]](#reference1) - See the "Possible parameters" session.
    
<dl>
    <dt>Reference:</dt>
    <dd id="reference1"><a href="http://raphaeljs.com/reference.html#Element.attr" target="_blank">[1] http://raphaeljs.com/reference.html#Element.attr</a></dd>
</dl>