Unused variable in `Option::parse`

[var-const]

The following variable appears to be unused: https://github.com/docopt/docopt.cpp/blob/400e6dd8e59196c914dcc2c56caf7dae7efa5eb3/docopt_private.h#L549

For context, I work on libc++ and we are considering enabling the unused variable warnings for some standard library types, including std::string. This produces a warning on your project and could potentially break some of the users (if they compile with warnings as errors), so we wanted to give you an early heads-up. The fix appears to be straightforward.