Use nopt to parse command-line options

[hsandt]

In order to add new options like "--newline-separator" in the future, without messing up with the existing options, I switched option parsing to nopt, which should make it easier to detect options anywhere in the arguments (currently, anything passed after --code, --file or --ast would be considered a "snippet" as part of the positional arguments).

This change is mostly retrocompatible for correct usage, supporting a few new cases like luamin "a = 5" --code (--code can be put after the positional arguments), and detecting compact forms like luamin -ch for luamin -c -h (obviously a bad example since it will ignore -c and just print help, but will be useful when we add more options, compatible with -c, -f and -a).