Optionally limit the note search by degree of separation

[joesfer]

The note graph grows unwieldy with large number of notes, many of
which are unrelated to the current selection and clutter the view.

This patch proposes a notion of context to the query, by adding an
optional limit to the degree of separation from the active note. The
previous behaviour of the plugin is preserved by defining a max degree
of 0 to mean "retrieve all notes".

[treymo]

This is great Jose! Thanks so much for your contribution!