Callback context

[rpkyle]

Dash.jl should support a callback_context method which will enable retrieving the inputs which triggered the firing of a given callback, and which also allows introspecting the input/state values given their names. It should only be available from within a callback, and it should present a warning message if a developer attempts to use it from outside of a callback.

This feature is described in more detail in plotly/dash#608 and plotly/dash-renderer#124.