Add DeckTape PDF exporter for slides presentation

[hermanliang]

Add feature of download slides as PDF file would be very useful for user who want to share their slides as a printable document.
DeckTape is a good candidate to integrate as a PDF exporter, because it supports reveal.js presentation framework.
I've tested the DeckTape on my computer (following the instruction) to export PDF from HackMD's example slides, and it works well.

./phantomjs decktape.js https://hackmd.io/p/slide-example slide-example.pdf

Thanks for considering my request!

[SISheogorath]

Currently, the API documentation for decktape is still completely missing. We should wait until a usable documentation exists.

[SISheogorath]

Seems like their API is still not documented. Closing for now.