Allowing customized collection to generate menu

TL;DR
* Added a piece of code
* Updated the doc
* Fixed a very minor issue in the readme
* Don't break backward-compatibility

I had the use case I wanted to render only a part of my document tree. I think custom collections are the way to do it.
That being said, I do **not** handle the error case (when the specified collection does not exist) since I don't know how to do it properly. Feel free to advise me or correct my PR to fix that.

Anyway, thank you for the plugin !