Add extra markings to specific cells

[ppintofilipe]

Hi Alicia!

Congratulations on this package. It's very interesting and full of very useful features!

I'm using it to plot some hypothesis tests (for differential analysis of genes), and I'd like to mark the cells with statistical significance (for example, by adding an asterisk to those cells or by adding a stroke with different colouring).

Is there any way of doing that?
Thank you!