This adds the capability for personal notes

In some use cases, it's nice to have the ability to add personal notes
to be displayed alongside the built in presenter notes. This commit
allows you to do this. Just create a short markdown file in `_notes` that
matches the complete pathname of the slide that you'd like to add notes to.

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Merge pull request #198 from binford2k/feature/personal_notes

This adds the capability for personal notes