magicbot: Add tunable update listener method

[auscompgeek]

This adds a way to add a callback for NT updates to tunables.

Ref: #133

[virtuald]

One objection I have with this implementation is adding an additional entry listener. As you add more of them it'll keep making callbacks more expensive (which is a problem with the current ntcore implementation... ).

[auscompgeek]

@virtuald Now that we have pyntcore, do you still have objections to this?

[virtuald]

Callbacks still have the same issue.