Unused variable in `Option::parse`

[var-const]

The following variable appears to be unused:

docopt.cpp/docopt_private.h

Line 549 in 400e6dd

std::string m = match[2];

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.