Questions regarding usage of `parse_arg`

Originally, the entire point of `parse_arg` was to provide the traits that types can implement to provide a canonical argument parsing (mainly dealing with `OsString`). The additional functions were scope creep and not really suitable for the crate. They have been removed in 1.0 (but kept in older versions using semver trick).

However, I've noticed you are using them. ~~Do you have any specific reason to not use `configure_me` instead? I intended for consumers to migrate to it but if there's a reason to not migrate I'd like to know if I should continue maintaining 0.x, create a different crate or something else.~~ Oh, I see what the reason is. You need to support unknown arguments. I will think about it and try to figure out a good solution. Please LMK if you'd like to use `configure_me` if it had support for unknown arguments.

In the meantime I'm fine maintaining 0.x, I don't think there will be much stuff going on. Also I think your code can be written more nicely, I might send a PR for that if I have extra time.