Support gitbook-like behavior

[egoist]

a gitbook mode, which means you can provide a summary.md to set the sidebar menu:

- [Get started](get-started.md)
- [Advanced](advanced.md)

Then we won't parse and set toc in the sidebar but instead use the custom menu, it's like the nav but moved to sidebar ^_^

[aleclarson]

Is there an existing way to customize the sidebar?