Fix issues with numering in ordered list

[blefevre]

This PR adjusts the ordered list parsing to correctly allow for interruptions in the list.
E.g:

1. Lorum
2. Ipsum
* Dolor
3. Sit Amet

Will now convert as expected instead of the numbering restarting at 1 after the unordered list.

This also adds the ability to use w:startOverride if present, so Documents can correctly alter the starting index of a list.

Closes #413