Add colour options

Colour options come from the command line (-c) or the EPLOT_COLOURS
environment variable or the hard coded default, in that order.

The format is '#RRGGBB[,(solid|dash|dot|dotdash)][,width][:#R2G2B2...]'.
Width defaults to 1 and the style to solid.  If the -c option is given
multiple times, the values are concatenated with ':'.