title: Configure default warn/error log with logback-classic/slf4j
body: <p>I am trying to configure logback-classic to handle the following situation:
  All messages originating from my.class with level debug or higher, should be sent to the stdout appender.</p>
  
  <p>And all error messages of level warn or higher, should <strong>also</strong> go to the altout appender no matter where they originate. But I can't seem to get it to work.</p>
  
  <p>I have tried this config, but the problem is that if I set additivity to true, then all messages logged to stdout are also sent to altout no matter their warning level. But if I set additivity=false then no messages originating from my.server are sent to altout even if they have a warn/error level.</p>
  
  <pre><code>&lt;configuration&gt;
  
    &lt;appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender"&gt;
      &lt;encoder&gt;
        &lt;pattern&gt;Stdout: %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n&lt;/pattern&gt;
      &lt;/encoder&gt;
    &lt;/appender&gt;
  
    &lt;appender name="ALTOUT" class="ch.qos.logback.core.ConsoleAppender"&gt;
      &lt;encoder&gt;
        &lt;pattern&gt;Altout: %d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n&lt;/pattern&gt;
      &lt;/encoder&gt;
    &lt;/appender&gt;
  
    &lt;logger name='my.class'  level='debug' additivity="false"&gt;
      &lt;appender-ref ref="ALTOUT" /&gt;
    &lt;/logger&gt;
  
    &lt;root level="warn"&gt;
      &lt;appender-ref ref="STDOUT" /&gt;
    &lt;/root&gt;
  &lt;/configuration&gt;
  </code></pre>