Add colors to the output of the screen

This commit aims to add colors to the output of the script for a better readibility and a nicer 'look'.

Colors are inspired by what's used in [libmakepkg](https://gitlab.archlinux.org/pacman/pacman/-/blob/v6.0.2/scripts/libmakepkg/util/message.sh.in):

This commit also brings some small code refactoring for a better readibility of the script.

Closes #81