fix: fixes a bug where using AppSettings::AllowHyphenValues would allow invalid arguments even when there is no way for them to be valid
fix: fixes a bug where using AppSettings::AllowHyphenValues would allow invalid arguments even when there is no way for them to be valid

Prior to this commit, using `AppSettings::AllowHyphenValues` would allow
ANY argument to pass, even if there was no way it could be valid.

Imagine a CLI with only a single flag (i.e. *no value*) `--flag`, but this setting
is set. The following was valid:

```
$ prog hello
```

This commit fixes that by creating an UnknownArgument error unless the
unknown argument/value in question could legally be parsed as a value
to a valid argument.

Closes #1066

clap-rs/clap,kbknapp/clap-rs,kbknapp/clap-rs,clap-rs/clap,clap-rs/clap