Add Powerpoint writer.

This imports the essential Powerpoint writer.

It works following the standard Pandoc conventions for making other
sorts of slides. At the moment, there are still these TODOs:

1. Syntax highlighting is not yet implemented. (This is difficult
   because there are no character classes in Powerpoint.)

2. Footnotes and Definition lists are not yet implemented. (Notes will
   usually take the form of a final slide.

3. Image placement and auto-resizing has a few glitches.

4. Reference powerpoint files don't work dependably from the command
   line. This will be implemented, but at the moment users are advised
   to change themes from within Powerpoint.

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