Create `as.ggplot.tmap`

[jospueyo]

First of all, thank you for this package!

I hoped that it would work with the maps created by {tmap} package. I often struggle to align them with ggplots. But it seems they are more complex than I expected. It would be really nice if you could implement a method for tmap class. I could create a PR with this but I don't know how to start, I'm not familar with the ggplot internals.