unknown option handler should be able to handle flags in --flag=value format

I realized there is an edge case in my previous pull request that wasn't covered, which is that if the flag is specified as a single string with "=" then the value itself doesn't appear in the parser state's args array. This should fix it by passing the parsed value from splitOption into the unknown option handler function.
I think this should be changed to "an Argument which specifies its value if set with an argument separator" since `splitOption` uses "=" or ":" under windows.