Allow override for --top-level-division option

[dimosr]

First of all, thanks for the library, it's really handy!

I noticed that it currently defaults to --top-level-division=chapter and I could not find a way to override this value to part, using the leanpub command, so I hacked the library locally to change the hardcoded value.

If you could provide an option for that (or in general to pass through parameters to the pandoc command), it would be really helpful.