NetteExtension: add all mailer options to defaults

Commit https://github.com/nette/nette/commit/2615fbab555bc1dea703a00e453b380b7cd0275f[2615fba] introduced bug where NetteExtension throws ```Nette\InvalidStateException - Unknown option nette.mailer.host, nette.mailer.username, nette.mailer.password, nette.mailer.secure.``` when defining any other option than ```smtp``` which was already defined in extension. Other sections can be afflicted too.