Add label/tags to `gggrid` plots

[Matt-BF]

Thanks for the great work on lets-plot! It's my go-to package for figures nowadays.

It would be great for gggrid to allow tagging of each sub-figure, similar to patchwork in R or plotnine. This is particularly useful for figures in scientific papers

If this is already implemented, let me know how I can use it, I couldn't find it in the docs. Thanks once again!

[alshan]

Good point! Currently, lets-plot lacks this feature.

[alshan]

See also labels param in plot_grid() of cowplot.