Add Toc commands.

Create a quickfix vertical window with the headers indented according to level.

Fundamental to navigate and understand the structure of large markdown files.

Test with `tests/multiple_headers.md`.

GIF screenshot:

![out](https://cloud.githubusercontent.com/assets/1429315/2559732/b9cfa208-b789-11e3-933a-40106c3559ac.gif)

The following plugin does something similar but I'm not sure if it supports quickfix navigation: https://github.com/matthias-guenther/tocdown/issues/5