add toc automatically if the markdown has so many ## header2

it is a NICE feature that TOCs are generated automatically when the depth of headers is a greater than 3 by default.

while [this markdown](https://github.com/downgoon/vertx-translation-chinese/blob/dev/core/Core.md), has no level-3 headers, but so many level-2 headers that readers always 'get lost' in some section.

**i guess depending on the amount of level-2 headers will also be an additional nice feature.** setting ``mindepth=2`` can meet this case, but ``Zero Configuration`` will be much more greater ?