Framework for JavaScript cells

[tdudgeon]

We need to define a general mechanism for how client-side functionality can be added. The critical factor here is that users must be able to add new cell definitions without adding anything on the server side.
We can use the various D3-based cells that we have underway or planned as initial prototypes, but this extends beyond D3-based visualizations.

There are various aspects to this that need to be considered, but it could look like this:

1. Cell definition allows javascript libraries that are needed to be specified
2. A class in one of the libraries is defined as the "entrypoint"
3. The "entrypoint" class provides the interfaces that define:

• the cell's inputs and outputs (or these are part of the cell definition)
• how the options are declared (or these are part of the cell definition)
• how the cell UI (e.g. D3 visualisation) works

There may be lots of ways this can be done, and this example may be be off the mark. Much needs to be considered. But something that achieves the key goal of not needing to add code on the server side is needed.

In addition we may need some sort of SDK to allow users to develop cells in an IDE.