Add note generator to panbeam for pdfpc

When using pandoc with panbeam to generate slides from markdown, we now also
generate speaker notes for use with pdfpc.  Comments of the following form
will now appear as speaker notes:
<!---
This will be speaker notes.
-->

We use some simple bash scripting to look through the markdown source and
determine which slide your comment should appear with.  Currently only works
with slide level 2.