Support presenter notes

Allow slides using the "note" directive to contain
presenter notes that are displayed only in the presenter
console.

Change-Id: I9230e7a72f39400aa5b02e76c16da0338ca764b5
Signed-off-by: Doug Hellmann <doug.hellmann@gmail.com>
Merge pull request #29 from dhellmann/presenter-notes

Support presenter notes