Add ability to generate Table of Contents fro Wikis

Customer / Requester: Big Consumer Electronics Company
Zendesk ticket: https://gitlab.zendesk.com/agent/tickets/7682

Requested feature

This customer would like to able to generate a Table of Contents for the Wikis that can be displayed on the main wiki page.

Related feedback item: http://feedback.gitlab.com/forums/176466-general/suggestions/4320328-enable-all-the-goodies-in-the-gollum-wiki-engine

Concrete questions / Next steps

@JobV what do you think?