Support Custom Chains

[nickbabcock]

Collectd allows one to write custom filters / chains. See fc_register_match and fc_register_target in filter_chain.h. While I would like collectd_plugin to support chains, I don't have a use case so I haven't set aside any time to solve this issue.

It may be best to add a couple methods to PluginManager:

fn matchers(_config: Option<&[ConfigItem]>) -> Result<Vec<Matcher>, Error> {
    Vec::new()
}

fn targets(_config: Option<&[ConfigItem]>) -> Result<Vec<Target>, Error> {
    Vec::new()
}