Allow customising the legends

[rcannood]

funkyheatmap added a way for customising the legends: https://funkyheatmap.github.io/funkyheatmap/reference/verify_legends.html.

Every item output by verify_legends() is then translated into a separate funkyheatmap (based on the same data structures) in https://github.com/funkyheatmap/funkyheatmap/blob/15327fb37fce4869f6389c807aad86e772948de4/R/create_legends.R.

Can we add this to funkyheatmapjs as well?

[mxposed]

Fixed in #25