Offer concrete repr for interactive expressions

The promise of `hvplot`'s `.interactive` API is that you can use the same API as pandas/xarray with widgets and so on. It would be useful if `.interactive` could be used to explore a parameter space for pandas/xarray expressions where the goal is to output an expression that can be used with pandas/xarray *without* hvplot.

This could be a repr where the widgets are replaced with concrete values. That said, *if* the expressions can have a real `repr` (which I suppose it can when using widget class is supplied) then that is a better repr as it would preserve the object more closely. At any rate, some way of getting the concrete expression (i.e. that works without hvplot) would be a nice feature to have.

Such a thing might even be a good tool for testing (e.g. eval this and make sure the non-hvplot version matches).