Add :view-callback for access to the vega view api

[metasoarous]

This solves a lot of problems, the biggest of which is making it possible to react to signal changes in the vega visualization, but also opens up all of the vega view api functionality to Oz users.

This issue effectively solves #13, though there is potentially some room for higher level options for reacting to signal changes and updating data spcifically.