[Feat] Use auto colours if cli option for colours is provided without any value

[eliasm307]

When using the cli, providing the -c or --prefix-colors option without any value uses the default value of "reset", ie the normal colour.

Since this is the same as not having any colours defined, I think it would be more useful if the option is provided without a value then it uses auto colours, as the user likely wants colours (otherwise they wouldnt use the flag). This could be a shorthand instead of having to do -c=auto

[gustavohenke]

Has been implemented by #581.