api(Arg::last): adds the ability to mark a positional argument as 'la…

…st' which means it should be used with `--` syntax and can be accessed early

Marking a positional argument `.last(true)` will allow accessing this argument earlier if the `--` syntax is used (i.e. skipping other positional args)
and also change the usage string to one of the following:

* `$ prog [arg1] [-- <last_arg>]`
* `$ prog [arg1] -- <last_arg>` (if the arg marked `.last(true)` is also marked `.required(true)`)

Closes #888