Add "selection" tool callback to a Bokeh figure

This adds a selection callback to be used with the box select or lasso select tools.

tool_selection(fig, callback, ref_layer)

Arguments

fig	figure to modify
callback	a callback to be applied to this tool - either a character string of javascript code or any one of debug_callback, shiny_callback, console_callback, custom_callback
ref_layer	name of the layer that the callback should be applied to