Add colour option for -p's slow test output

Created by: JoshCheek

Worked on with @cdemyanovich during waza at @8thlight

When running RSpec with the --profile option, slow tests are displayed in red. This causes me to think my tests fail!

This patch introduces the --slow-color (akas: --slow-colour, -s) option to allow the color to be changed. It defaults to cyan.