Add article or vignette explaining targetting of colour, fill and alpha

[aphalo]

geom_point_s(), geom_label_s(), geom_text_s(), geom_label_pairwise(), geom_text_pairwise(), geom_table(), geom_plot(), geom_grob(), geom_table_npc(), geom_plot_npc() and geom_grob_npc() have formal parameters colour.target and alpha.target. Arguments passed to these prameters work consistently while each geometry accepts a subset of the whole set of target names. A common explaination with examples would make a useful article.

Use of arrow() should be also explained in this article as it is also relevant to all these geometries.