'Control' input type

[mvahowe]

Right now all data must come from a source. This is very orthogonal but also clunky when, eg, the input is simply a string from an enum to specify the type of diff. In such cases we could dispense with the source step and, instead, let the user provide the content directly in the transform, either as a text input or as a select.

This would mean changes to the transform templates, and also to runCallback.