Partial support for optional arguments

[halfflat]

If option argument parsing (or end of args testing) is permitted to fail without throwing an exception, we could support optional arguments and options that take different sorts of arguments with e.g.

optional<int> v;
to::opts options[] = {
    { v, "-f", to::lax },
    { to::set(v, 1}, "-f" }
};

trying to parse an integer argument to an option -f to assign to v and then, if it fails to parse an integer, instead just sets v to 1.

Caveats:

• String arguments are going to be greedy, and would gobble e.g. -- as an argument even if to::lax is given, based on the approach above.
• The order of options presented to to::run is crucial for this to operate as expected.