title: Boost logging set output level
body: <p>I was wondering if there is a something similar to <code>FLAGS_stderrthreshold</code> that only outputs the level that is more severe than the flag in boost library. For example, the code I have below prints everything to the console. Can I set the level so that it only prints warning, error and fatal?</p>
  
  <pre><code>#include &lt;boost/log/core.hpp&gt;
  #include &lt;boost/log/trivial.hpp&gt;
  int main() {
      using namespace  boost::log::trivial;
      boost::log::sources::severity_logger&lt; severity_level &gt; lg;
      BOOST_LOG_SEV(lg, trace) &lt;&lt; "A trace severity message";
      BOOST_LOG_SEV(lg, debug) &lt;&lt; "A debug severity message";
      BOOST_LOG_SEV(lg, info) &lt;&lt; "An informational severity message";
      BOOST_LOG_SEV(lg, warning) &lt;&lt; "A warning severity message";
      BOOST_LOG_SEV(lg, error) &lt;&lt; "An error severity message";
      BOOST_LOG_SEV(lg, fatal) &lt;&lt; "A fatal severity message";
      return 0
  }
  </code></pre>
  
  <p>Actual Output: </p>
  
  <pre><code>[2014-03-13 22:21:55.734957] [0xc00001d6] [trace]   A trace severity message
  [2014-03-13 22:21:55.735957] [0xc00001d6] [debug]   A debug severity message
  [2014-03-13 22:21:55.735957] [0xc00001d6] [info]    An informational severity message
  [2014-03-13 22:21:55.735957] [0xc00001d6] [warning] A warning severity message
  [2014-03-13 22:21:55.735957] [0xc00001d6] [error]   An error severity message
  [2014-03-13 22:21:55.735957] [0xc00001d6] [fatal]   A fatal severity message
  Press any key to continue . . .
  </code></pre>
  
  <p>Desire Output:</p>
  
  <pre><code>[2014-03-13 22:21:55.735957] [0xc00001d6] [warning] A warning severity message
  [2014-03-13 22:21:55.735957] [0xc00001d6] [error]   An error severity message
  [2014-03-13 22:21:55.735957] [0xc00001d6] [fatal]   A fatal severity message
  </code></pre>