title: twisted log level switch
body: <p>Is there any way in Twisted how to change logging level of messages which should be logged?</p>
  
  <p>I am using three levels in project:</p>
  
  <pre><code>log.msg('agent nr.1 has free slots', logging.DEBUG) # debug message
  log.msg('agent nr.1 has free slots') # info message
  log.err('agent nr.1 has free slots') # error message
  </code></pre>
  
  <p>And I configure logging in this way:</p>
  
  <pre><code>from twisted.python import log
  from twisted.python.logfile import LogFile
  
  logfile = LogFile("someFile.log", '/some/path/', rotateLength=1000, maxRotatedFiles=100)
  application.setComponent(log.ILogObserver, log.FileLogObserver(logfile).emit)
  </code></pre>
  
  <p>But I need set which messages should be logged (e.g. only info and error messages, no debug). How to do it?</p>