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.

withing -> within

The previous logic here was backwards, and did not allow the user to override `options` in the mathjax config structure.
This makes it match how the `startup` and `tex` fields are merged.