add CDATA support to include raw html if options.cdata==true

Add CDATA to include any raw html:
```
normal markdown text
<[!CDATA[
  <script>
    window.alert('allow script in markdown');
  </script>
]]>
continue markdown...
```

CDATA is disabled by default for security reasons, and can be opened in options:

```
markdown.toHTML(s, 'Gruber', { cdata: true });
```