update markdown default notes separator to ignore inline occurances of 'notes:', closes #1915, closes #2762

```js
Reveal.initialize({
    markdown: {
        defaultOptions: {
            verticalSeparator: '\n--\n`,
        }
    }
})

If the markdown contains something that is indented by more that the
`leadingTabs`/`leadingWs` then extra white space is incorrectly removed.
ie the following example:

```
    <section data-markdown>
    some text
       indented text
          more indented text
    </section>
```

would result in the following markdown:

```
some text
   indented text
  more indented text
```

We can work around this problem by using a function to generate the
replace value.

Add an "animateLists" option to the markdown plugin that animates all
lists by default.

Don't waste time constructing a renderer then override it if one is
passed into the options object.