added sanitation to the slideMode in frontmatter

This should prevent the issue mentioned in https://github.com/hackmdio/codimd/issues/1648

Specifically left out are
- dependency (user can't really include anything anyway, because CSP forbids most domains)
- autoSlideMethod (nothing our users should be able to change as they won't write JS to be affected by this)
- keyboard (this let's users write arbitrary code and seems therefore to problematic)

See:
https://github.com/hakimel/reveal.js/blob/3.9.2/README.md#configuration
Signed-off-by: Philip Molares <philip.molares@udo.edu>

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