Update Tracing component's filter after loading a config file

[aawsome]

I want to define log levels in in a config file. To do so, I was able to get the tracing component by its ID, cast it and use the reload_filter method. This feels however awkward.

I propose to impl Component's after_config() for trace::component::Tracing.
(This is related to #332 as Component is currently derived.)

Moreover, I propose to add a method update_tracing_config(&self) -> Option<trace::Config> to Config (with default impl returning None), such that the after_config() for the Tracing component could look like:

    fn after_config(&mut self, config: &A::Cfg) -> Result<(), FrameworkError> {
        if let Some(config) = config.update_tracing(){
            self.reload_filter(config.filter)
        }
        Ok(())
    }