Support gtable/grob object

[thomasp85]

It would be nice if vdiffr would support gtable (and potentially all grobs) natively in expect_doppelganger so that expressions that returned a gtable would get handled automatically. While I personally need it for visual testing in gganimate I guess it would also be beneficial to others relying on gtable manipulation for their functions

[lionel-]

This makes sense. If you need it right away I'd welcome a PR. Only the github version of vdiffr is usable anyway, until freetypeharfbuzz gets reviewed by CRAN.

[thomasp85]

I'll carve out some time for it