add support for nested lists

[stutrek]

Thank you for this library, it's the best regex barf I've worked with in years.

This PR adds support for sublists in the standard markdown way:

- lists
  - within lists
- also
  1. different types of sublists

I am embarrassed that it changes the output of one test, but it's only a whitespace change so rendered HTML should remain the same.

Fixes #104 Successor to #110