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.

So far, multiple notes per slide are only supported if they are
attached to fragments (without fragments, only the first aside notes
element on a slide is displayed).  With this commit, the contents of
all notes on a slide are displayed (except for fragments with notes,
for which, as before, only each fragment's first/single note is
displayed).