Hook for callback upon document init

For any kind of plotting that shall start without having the user doing anything in the first place, like an animation of data, or periodic update from (time based) calculations, without having to use a bokeh server, a hook that triggers a callback at page load would be needed.

[bryevdv]

So I'm about to look at a callback when the Bokeh document is initialized. Will that suffice? I'm not sure page load is workable, generally, given context like embedded server apps, and the jupyter notebook.

Assuming so,I will use the PR to refer to and close this issue.

Yes, that will be sufficient. Just so that the callback gets called after the first time the plot has been rendered.

[bryevdv]

The recent events work by @jlstevens should have layed most of the foundation for this.