Please add support for speaker notes

[macu]

I am seeing open issues about slide notes and viewers like Keynote, and indeed adding notes causes a format invalid error message in this app for me. E.g. #83

The real issue is that speaker notes should be simple to add to a slide, as plaintext blocks. See here about speaker notes: https://support.microsoft.com/en-us/office/add-speaker-notes-to-your-slides-26985155-35f5-45ba-812b-e1bd3c48928e

It would be great to have a simple method on the slide class to add speaker notes.

[Progi1984]

http://officeopenxml.com/prNotesSlide.php