See docs for more details on 'temperature gradients', which can be
turned on with the -t switch on graphs. Also improved parsing of
options with $execgraph and $execigraph, but may cause some breakage.
As a side effect, font and colour specific stuff got outsourced, too.
This is because the apropriate functions are used by conky.c as well as
specials.c, so they should be kept on their own.
In the long term I hope for positive impact on the X11 integration mess.
(Take e.g. the mass of X11 ifdefs cluttering conky.c.) Though this
commit contains no optimisations in this direction, just plain
outsourcing, to ensure minimised (intended: none) changes to conky's
interface behaviour.
