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 that this should be changed too since there are many ways to define this kind of argument e.g. its applicable to short arguments too.
How about 
`Argument represents the argument value of an option that was passed using an argument separator.`