Allow forcing color output via environment variable SSHKIT_COLOR.

Hi,

on Jenkins you can install the ANSI color plugin. But even with this plugin installed `$stdout.tty?` would never return true. So I have introduced a new environment variable to force color output from the outside.

Another thing I've notice is that the colors will leak to the next line. So you'd need to put a reset ANSI code at the end of each line. But I'll have a look into that separately (maybe).

Cheers,
plu