keep radiated labels within plotting window

[corybrunson]

Currently, geom_text_radiate() adjusts hjust internally to shift text away from arrowheads, but the plotting window does not account for this.

library(ordr)
pca <- prcomp(USPersonalExpenditure, center = FALSE)
pca_ord <- as_tbl_ord(pca)
pca_ord <- confer_inertia(pca_ord, c(.5, .5))
ggbiplot(pca_ord, aes(label = .name)) +
  theme_bw() +
  geom_v_vector() +
  geom_v_text_radiate() +
  geom_u_point()

[corybrunson]

This has been resolved in the best way currently possible in 5c5b9d1, by allowing clip = FALSE to be passed to ggbiplot() and thence internally to coord_equal().

[corybrunson]

The expand_limits() layer seems to resolve this issue more elegantly, i.e. without conflicting with the scale_*_continuous() calls inside ggbiplot(); though ultimately those internal calls should be replaced with more direct steps that don't conflict even with downstream scale_*_continuous() layers.

[corybrunson]

This issue may be fixed via #34.