Add support for window selection via window-numbering

Hi Pei,

This PR adds [window-numbering](https://github.com/nschum/window-numbering.el) support.

If `window-numbering` is available then it is possible to quickly choose the window where to open a node.
Example:
To open a node in window number 2 press: <kbd>2 RET</kbd>

This has been tested in `spacemacs` with and without `windows-numbering` installed.

Cheers,
syl20bnr