Support for Multilevel documentation

[rohajda]

Hello, Your tool is great help and almost perfect. My only issue is with mkdocs option when yaml file is written following way

pages:
- Home: 'index.md'
- User Guide:
    - 'Writing your docs': 'user-guide/writing-your-docs.md'
    - 'Styling your docs': 'user-guide/styling-your-docs.md'
- About:
    - 'License': 'about/license.md'
    - 'Release Notes': 'about/release-notes.md'

in such case generated pd file is without header level 1 for User Guide and About causing result PDF to not be structured as I want.

Home is correct as there is index.md file but if there is not mardown file it will not be placed to generated file.

[twardoch]

@rohajda Please check out my fork https://github.com/twardoch/mkdocs-pandoc/ which has some merged commits from other users, including one that should resolve your problem.