v.label / r.contours options to control label spacing

[stevensj]

r.contours makes nice contours, but when you plot them in ps.map the labels aren't as nice as in GMT. One of the most useful controls in GMT is one that lets you decide the spacing of the lables.

It would be nice if either r.contour could be adjusted to include an option that specified where on the vector lines that it creates the labels should be placed, or if v.label could be adjusted to have a "contour" flag that puts extra labels at a given spacing along vector lines.