let users set colors and labels

[lvaudor]

Hi!

Thanks for this very useful package. We have been using it a lot recently and would like to be able to change the clusters' labels and colors when using rainette_plot(). The rationale behind this is that we use the rainette package's results for further analysis and would like to show the clustering's results once and for all (with final, "interpreted" labels for clusters, and custom colors).

I have implemented this in this PR , mainly through new argument "colors" and possibility that previous argument "cluster_label" is actually a vector of size k (number of clusters). I have put an example in the plot_rainette() function examples section. As far as I have seen it runs smoothly and does not change anything in case cluster_label has size 1 and custom colors are not provided.

I hope it can be useful, but please feel free not to include it if you do not find it relevant.
Cheers,
Lise

[juba]

Merged, thanks !