Use a gem for generate colored output, this fixes #47

The intent is to use the gem `term-ansicolor` since all the other gems (colorize, colored, rainbow) uses it behind the scenes.
A new module `Pacto::UI` was created, it's responsibility is to transform a string into a colored string. By now is inheriting from `Term::ANSIColor` but in the future we can change the implementation without affecting the rest of the code.