Add 'literal' tag to make content in the tag pair be taken as literal and not be parsed

When you ejs template contains literal strings <% or %>, it will be recognized as delimiters by ejs engine,where it is not the template author's intention and it probably cause en parse error. We need add support to make some content (especailly string equals to delimiter ) be treated as literal and not parsed by ejs.
I  add such support as a literal tag.Its usage is as follows:
```html
  <%literal%>
    <span>this span contains delimeters <% %> , it should be wrapped by literal tag</span>
  <%/literal%>
  <script type="text/javascript">
    var template="<%=title%>";
    ...
 </script>
```