title: Change timestamp format used by env_logger
body: <p>The following minimal example</p>
  
  <pre><code>#[macro_use]
  extern crate log;
  extern crate env_logger;
  
  fn main() {
      std::env::set_var("MY_LOG_LEVEL", "info");
      env_logger::init_from_env("MY_LOG_LEVEL");
      info!("hi");
  }
  </code></pre>
  
  <p><a href="https://play.rust-lang.org/?gist=3ca25a6aa9b279abef5c8de477c9b2e7&amp;version=stable" rel="nofollow noreferrer">outputs</a>:</p>
  
  <pre class="lang-none prettyprint-override"><code>INFO 2018-02-18T09:59:20Z: playground: hi
  </code></pre>
  
  <p>I would like to use a different format (<code>"%Y-%m-%d %H:%M:%S%.3f"</code>):</p>
  
  <pre class="lang-none prettyprint-override"><code>INFO 2018-02-18 09:59:20.123: playground: hi
  </code></pre>
  
  <p>How can I achieve this?</p>