Scriptable context to align to CHART.JS

[stockiNail]

Having a look to mode and overScaleMode, I see they are scriptable but the created context passed to callback seems:

1. missing of type attribute
2. is not persistent therefore every invocation creates new object

I was thinking to propose a PR to add the scriptable column to the options but I'm not sure if this is something that we want, because also the defaults are defined without the descriptors.

[kurkle]

It is something I've thought of, but have been postponing until critical bugs are fixed