- removed darksage, lightsage, sage which are colors we added in
   1ac590e  These colors are now
   included in the XKCD mapping
 - correct sandybrown which mpl had as '#FAA460' but should be '#F4A460'
   according to the w3 group, wikipedia, the copy of rgb.txt bundled
   in the emacs source, and the mapping that we had bundled with AGG for
   a while.  The original value goes back to
   ec45d17

 - convert the single-letter colors to hex in source
 - single path for looking up string -> hex -> rgb values

 - prefer the CSS4 str -> hex values because they are the 'standard'.
   The xkcd colors might be 'better', but we are not the right place to
   lead that change.  If users use CSS/HTML/X11 color in mpl they should
   expect it to match the same color set in html for embedding mpl
   figures into websites and such.

 - Provide entries in XKCD dict of all names (with and with out spaces)
   prefixed with XKCD.  Thus to get XKCD's version of blue use
   `color='xkcdblue'

The hex representations are not exact which resulted in
very small slight color changes.

ColorConverter lowercases everything on the way in.