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).

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.