Instead of finding the notebook at the wikiroot, we directly change the
current working dir to the notebook for the runtime of the zk command.
This has a couple advantages: Any other commands executed will also pick
up the correct directory, and creating notes in a specific directory
also just _works_ by giving a directory in the notes dir even if we are
somewhere else.
We use a custom zk indirection which ensures that any short alias for
interacting with the zk wiki will actually act on the globally assigned
wiki (by ensuring notebooks dir is "$WIKIROOT").
Local wiki requires the '$WIKIROOT' env var to be set, pointing to the
root of the (zk) wiki. So we only create associated aliases if the wiki
actually exists on a machine.
The note dir is in fact our `$WIKIROOT` so we set it to that by default.
Also took the chance and added a small 'cd' command into the note dir,
using `ncd`
