Scan Logback configuration files

[sebastiankirsch]

Given the following Logback configuration

<configuration>
  <appender name="myAppender" class="de.is24.deadcode4j.UsedClass"/>
  <root level="debug">
    <appender-ref ref="STDOUT" />
  </root>
</configuration>

de.is24.deadcode4j.UsedClass is not recognized as class being in use.