opt_present with invalid value should not panic

[mqudsi]

If matches.opt_present is called with a not-defined option, that should either be a compile-time error (somehow) or it should return false; currently it panics instead.

[KodrAus]

The current behaviour of panicking highlights an issue in the program itself where it's trying to find a value that'll never return true. I think this is maybe a little surprising but is a reasonable way to identify a possible bug.

We should note in the crate docs that the method will panic if it's given an option that doesn't exist.