Add labels.subset argument to plot only some labels

[tpq]

Hi,

Thanks for making a great package. It saved me lots of time!

I work with high-dimensional data. Plotting all arrows makes the figure too crowded. I modified your code to plot only a few arrows at a time (toggled via the labels.subset argument). Please accept this pull request if you think others might find this feature helpful too.

Cheers, Thom