Allow -Ccolor1,color2, etc to also take +h for hinge

[PaulWessel]

For an odd number of colors we may use the +h modifier when building a CPT from colors. E.g.,

gmt makecpt  -Cblue,white,red+h7.5 -T0/10
0	blue	7.5	white	L
7.5	white	10	red	L
B	black
F	white
N	128

Since it is more of a new feature than a bug fix I suggest we leave this as WIP until 6.4 is out.