Fix error output of the used option name
Fix error output of the used option name

If you use --formatter=foo, Option::name is "--formatter=foo"
instead of simply "--formatter". We add a small auxiliary
method that returns the correct name (as std::string).

ogdf/bandit,ogdf/bandit,joakimkarlsson/bandit,joakimkarlsson/bandit,joakimkarlsson/bandit