support <textarea data-template> `some code` </textarea>

Closes #1163, #1495.

In html content, marked allow `Inline-Level Grammar` but not `Block-Level Grammar`, so when I write following:
```
note:
* a
* b
* c
```
it become:
```html
<p><em> a
</em> b</p>
<ul>
<li>c</li>
</ul>
```
unbelievable!

by changing the tag before html is parsed and restoring it before markdown parsing

see https://github.com/hakimel/reveal.js/issues/929#issuecomment-80738837

add global flag back in for whitespace removal, without it most of the content in example.html does not work #682

renamed markdown attributes for clarity and consistency: data-vertical -> data-separator-vertical, data-notes -> data-separator-notes

- on slides: .slide x=y
- on elements: .element x=y

Those are detected in html comment, in order to not be visible
if a markdown is edited in an html editor.