Support for run monitoring visualization extensibility

[djf604]

If a developer wants to add his or her own plots, data structures, etc to the visualization output it would be best to be able to add them directly to the Parsl supported data viz.

My idea is that there could be a hard-coded skeleton, then customizable "blocks" that can be added/removed as the developer desires. Parsl can provide a handful of useful plot blocks and a default arrangement, but the developer can easily add custom blocks as well in whatever order.