and make muting work for negative values

This allows us to use different colours for different timers,
which will be useful when we add a pacing timer.

This will allow us to reuse the display code for displaying
an additional pacing timer.

cdn.mathjax.org is shutting down: https://www.mathjax.org/cdn-shutting-down/

This changes the default MathJax URL in the math plugin, as well as
references in README.md and test/examples/math.html

Use window.callPhantom icw page. onCallback to wait for pdf-ready event.
From: http://stackoverflow.com/a/28925479/129269

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

Closes #1163, #1495.

When a slide has several fragments it could be convenient to define a note for each of them. In this case we need to show only this specific note defined in a fragment and not others. General note of a slide shouldn't be also shown, as a more specific one should have greater relevance in this case.

Switching a `display: block` element to `display: inline-block` allows calculating the bounds based on the contents of the div rather than the entire container (which is often `width: 100%`).

This provides a much more natural zoom, especially for paragraphs and code examples.

Closes #1392.

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!