Example of using IterationManagers.jl with PlotlyJS.jl to update plot

[cc7768]

Still needs some clean up, but the concept is here.

[sglyon]

Cool!

This will be extra great when we can compose managers and states.

It would be best if we could not replace the entire array every time we want to add a single point. What if we try something like this:

• Add a field to the manager that specifies the maximum number of points we want to every be displayed on the plot. Maybe maxpoints::Int or something like that
• Change plot_update to take and return scalars instead of vectors. I know this was just an example function, but let's try to get the example right!
• in update! lets call extendtraces!(mgr.p, 1, mgr.maxpoints; y=Vector[[new_point]]) instead of restyle!. This will leverage plotly's efficient mechanism for adding data to a trace attribute rather than just replacing the whole attribute.
• We can just let iter_hook and post_hook be functions that return nothing.