Request for plotly_webglcontextlost event handler

[markovist]

I make use of WebGL graphs and will sometimes lose the context when interacting with them heavily (i.e. making multiple large scattergl selections consecutively).

It would be very useful to have a handler for the plotly_webglcontextlost event so that users can choose to handle these errors gracefully and redraw their graphs if necessary. It appears to be a fairly straightforward addition.

[dmt0]

@markovist you're welcome to submit a PR :)
You can check the last 2-3 PRs that were merged for a template.