default label to name in text radiate geom (and maybe axis geom)

[corybrunson]

Usually a user will want to label radiating arrows with associated names, which will usually have been extracted from the ordination model as the name annotation column of the appropriate matrix. Almost never will a user want to label them with anything else except a variable they've manually constructed, in which case explicitly passing it to label will not only be necessary anyway but also make the code read more clearly. So GeomTextRadiate$default_aes should include label = name.

This should also perhaps apply to GeomAxis.

Finally, it may be appropriate to combine GeomVector and GeomTextRadiate into a single geom with distinguishable aesthetics, much as GeomAxis generates axes, tick marks, tick text, and axis labels. It would be possible to generate vectors without radiating text and radiating text without vectors, but this would require an explicit directive, e.g. label = "". Feedback is welcome on this.