Need support for clientside callbacks #129

Dash for Python provides an interface for describing data transformation relationships between components in JavaScript (plotly/dash/issues/266, plotly/dash/pull/672, plotly/dash-renderer/pull/143).

Updates which happen via this mechanism are triggered entirely client-side, without passing information over the network. Significant performance gains are possible relative to @app.callback functions in Python or app$callback functions in R.

Dash for R should offer support for so-called "clientside callbacks", just as Dash for Python does.