title: winston-daily-rotate-file prepend option not working
body: <p>I have my winston setup as follows</p>
  
  <pre><code>// Config import.
  var config = require('../config');
  
  // Library imports.
  var winston = require('winston');
  
  var logger = new(winston.Logger)({
      transports: [
          new(winston.transports.Console)({
              level: 'debug',
              colorize: true,
              timestamp: true,
              json: true,
              showLevel: true
          }),
          new(require('winston-daily-rotate-file'))({
              prepend: true,
              level: 'debug',
              colorize: true,
              timestamp: true,
              filename: 'log.txt',
              maxSize: config.log.maxSize,
              json: true,
              prettyPrint: true 
          })
      ]
  });
  </code></pre>
  
  <p>It logs to the console fine - as expected.
  It logs to file fine - as expected.</p>
  
  <p>However, the filename has the date tacked onto the end of the filename, not the beginning, like the <code>prepend</code> option would suggest.</p>
  
  <p>Is anyone able to offer any insight into this?</p>