Modify: remove feature "ignore_list"
This commit removes a feature "ignore_list". Use new feature
"nodup" instead of "ignore_list".
With this change, any value returned by a Proc registered by hook
that is not true, false, nil, etc. will now be converted to a
string and output.
----
$ seq 1 3 | ./outvoke.rb -q -e 'hook("stdin") { _1.body.to_i * 2 }'
2
4
6
----