add geom_plate to fit into ggplot workflows

[Swarchal]

Would be nice to create a custom geom to fit directly into ggplot2 code, although this will require working with the mess that is ggproto.

Example:

ggplot(df) + 
    geom_plate(aes(well = well, fill = values), plate = 384) + 
    facet_wrap(~plate_id)