geom for text labels at intersections of affine subspaces and plot window

[corybrunson]

It's currently difficult to overlay legible labels over geom_*_axis(). A natural scheme would be to position them just outside the plotting window, as though they were tick mark labels (but without tick marks).

[corybrunson]

This arguably violates the spirit and conventions of ggplot2. A suitable alternative may be to label the axes close to their intersections with the plotting window—for each axis, the intersection farther from the origin—by default parallel to the axes and shifted perpendicularly from them, as with geom_*_axis_text().

[corybrunson]

For future reference, this could be resolved differently if the clip parameter could be differently set for different plot layers.

[corybrunson]

Partially addressed in 51810ab.