Make rendering use optional dependencies

[pydanny]

Background

If mistletoe isn't installed, then the error is captured while a text string is returned. There is no option to use other renders. This makes mistletoe a 'magic dependency'.

What this PR does

1. Attempts to use mistletoe as the default
2. Provides the ability to use other renderers
3. On failure, provides a more clear message to the logs

[jph00]

We definitely don't want to make it a dep, since it's a large requirement for folks that don't need that one function. How about instead if the import fails we raise an error if the function is called?

[jph00]

I might be missing something, but I don't see how this would work -- e.g. markdown_func is used, but it's not actually set by the is None conditional AFAICT?

Add some examples/tests to the NB so we can be sure it's working correctly.

Since you've changed the render_outputs params to be docments-compatible, you should now properly document all the params (since a table will now be added to the docs with this output). Make the documentation of defaults more concise, and add explanations of every param.