add errorbars color

To John Hunter, John, I think this may be quite a common

    > wish. I usually want my errorbars to have different
    > properties to the default; not just colour but also a
    > different weight from the main trend line. Perhaps this is a
    > good candidate for factoring into the .matplotlibrc file and
    > maybe for extra format specifiers in the errorbar call. I'd
    > rate it an extremely low priority though, since you can
    > already achieve it as in the example. Alternatively, adding
    > this to a code sample could be useful to others. regards,
    > Gary

I think making this an rc setting is overkill because it is so
narrowly focused. I also don't think we need to do a full blown
format string (who wants dash dot error bars?).

A color arg would clearly be useful. As for line weights and other
props, I can't think of an elegant way to do it in the errorbar
signature because of the existence of the main line and the error
lines. You *could* do

  errorbar(blah, blah,
           lineprops = {'color' : 'r', 'linewidth' : 1},
           errprops = {'color' : 'k', 'linewidth' : 2})

but I don't think this is any easier, clearer or cleaner than just

  line, errlines = errorbar(blah, blah)
  set(line, 'color', 'r', 'linewidth', 1)
  set(errlines, 'color', 'k', 'linewidth', 2)

So I think an errcolor arg and example showing how to customize the
errorline properties from the lines returned by errorbar is the right
compromise. Are you volunteering :-)?