Add an option to force color mode in CLI

Created by: sebastien-prudhomme

Use case: continious integration with GitLab CI and Arachni docker image.

No TTY exists when launching Arachni in such environment, so no colors are used in the output (see print_color function in ui/cli/output.rb).

As GitLab is able to display terminal colors, add an option to force color mode even is there is no TTY.