Add bad argument list to user error when possible

[benfleis]

Addresses https://github.com/docopt/docopt.go/issues/37

This allows Parse callers to emit the list of unexpected arguments in some common cases, e.g., extra arguments, out-of-order arguments.

[fenollp]

@benfleis Still interested in making this work?

[benfleis]

@fenollp I may take a look at it, but am pretty far removed from this -- I haven't done go work in over a year. Feel free to take my diff and rebase it up to speed :)

[benfleis]

Dead.