Colourize the ping graph in curses.

The display used, .123abc> is nearly completely unreadable at a glance.
We can use colours, as the only terminal from last 25 years I'm aware of
that did not support colours was Win3.1/95 telnet.exe, and even that did
degrade to monochrome rather than to garbage.
Merge pull request #50 from kilobyte/master

Colourize the ping graph in curses.
I've been thinking about making this an option (i.e. you can disable this on the command line). 
Decision: No, set your terminal type to something that doesn't support colors if you don't want colors.