adapt ggraph textAlongGrob for axis and isoline labels

[corybrunson]

The labeling control of ggraph's edge geoms derives from the special textAlongGrob() function. Eventually it might become part of a minimal helper package, but for now it should work verbatim—with attribution!—with GeomIsolinesText, GeomAxisText, and GeomAxisLabel.

[corybrunson]

As suggested here, this might also be used with GeomTextRadiate.