Possible API change for mod_plot()

We could make the syntax more like `ggformula` if we specified non-default uses of the variables more like this:

```r
mod_plot(model, x = ~var1, color = ~var2, facet = var3 ~ var4)
```

This could allow for using other attributes (shape, alpha, linetype) too.

I haven't looked to see how hard the implementation would be.