Extend twistd's --logger to support passing arguments to the observer constructor

[itamar]

twistd's --logger option allows specifying a custom log observer. It would be good if the observer could be customized. Options:

1. Extend --logger f.q.p.n so that if you append colon, arguments after that are passed to function, similarly to e.g. endpoint string definitions syntax. So --logger f.q.p.n:x=y.
2. Add --logger-options.