Custom handler for extra flags

[rivertam]

I really like the idea of this library, but I'd prefer to think of flags as a set of common options available to many, but not necessarily all, programs within an ecosystem.

So when a flag happens to not compile into a particular binary, the set of flags (which basically act as an informal set of configurations) will suddenly crash the program with "Unrecognized flag: -u" or whatever the flag is.

I get why a lot of programs would prefer this behavior, but I'd really like the option to just ignore extra flags, or maybe print them as a warning or something.