title: Log4j2 Appender attributes with strict xml config
body: <p>I'm using log4j2-beta9 and want to configure it using a log4j2.xml in strict mode.
  My issue is: how do I specify attributes that are not in the shipped schema file?
  An Example:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8" ?&gt;
  &lt;Configuration
      status="DEBUG"
      strict="true"
      monitorInterval="5"
      name="TestingAttributes"
      verbose="true"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="Log4j-config.xsd"&gt;
      &lt;Properties&gt;
      &lt;/Properties&gt;
      &lt;Appenders&gt;
          &lt;Appender
              type="Console"
              name="SYSERR"
              target="SYSTEM_ERR"&gt; &lt;!-- cvc-complex-type.3.2.2: Attribute 'target' is not allowed to appear in element 'Appender'. --&gt;
              &lt;Layout Type="PatternLayout"&gt;
                  &lt;Pattern&gt;%date{dd.MM.yyyy HH:mm:ss,SSS} %5p %logger %m%n&lt;/Pattern&gt;
              &lt;/Layout&gt;
              &lt;Filters&gt;
                  &lt;Filter
                      type="MarkerFilter"
                      marker="FLOW"
                      onMatch="DENY"
  
                      onMismatch="NEUTRAL" /&gt;
                  &lt;Filter
                      type="MarkerFilter"
                      marker="EXCEPTION"
                      onMatch="DENY"
                      onMismatch="NEUTRAL" /&gt;
              &lt;/Filters&gt;
          &lt;/Appender&gt;
      &lt;/Appenders&gt;
      &lt;Loggers&gt;
          &lt;Root level="debug"&gt;
              &lt;AppenderRef ref="SYSERR" /&gt;
          &lt;/Root&gt;
      &lt;/Loggers&gt;
  &lt;/Configuration&gt;
  </code></pre>
  
  <p>Notice that I want to set the appender to have the target <code>SYSTEM_ERR</code> but the attribute is not allowed in strict mode.</p>
  
  <pre><code>target="SYSTEM_ERR"&gt; &lt;!-- cvc-complex-type.3.2.2: Attribute 'target' is not allowed to appear in element 'Appender'. --&gt;
  </code></pre>
  
  <p>I could always edit the <code>Log4j-config.xsd</code> and allow that attribute there but that would be kind of wrong also because not all appenders have a <code>target</code> attribute.</p>
  
  <p>As searching the web didn't help me so far, I'm asking you:
  Is there anything I'm missing in configuring Log4j2 in strict XML mode?</p>
  
  <p>Do I have to "patch" the XMLConfiguration and the schema file and commit a change to log4j or is there another way besides not using strict mode?</p>
  
  <p>Thanks in advance.</p>