Add class parameter typesdb

Some collectd modules (e.g. dbi or postgresql) can use user defined types to collect database results. In order to define such a type one either has to modify the supplied `types.db` file or add a local `types.db` file to the configuration. The new class parameter `typesdb` takes an array of file names to allow multiple `types.db` files in addition to the one supplied by collectd.