title: How to make a custom json formatter for winston@3 logger?
body: <p>I'm trying to make a custom json formatter for winston v3. </p>
  
  <p>Here is a single file with demo:</p>
  
  <pre><code>const { createLogger, format, transports } = require("winston")
  const { combine, timestamp, prettyPrint } = format
  
  const logger = createLogger({
    format: combine(timestamp(), prettyPrint())
  })
  
  logger.add(new transports.Console())
  
  logger.info("test message", { data: { a: 1, b: [1, 2, 3, 4], d: new Date() } })
  
  try {
    throw new Error("I'm an error!")
  } catch (err) {
    logger.error(err)
  }
  </code></pre>
  
  <p>It prints:</p>
  
  <pre><code>{ data: { a: 1, b: [ 1, 2, 3, 4 ], d: 2018-07-21T08:59:27.958Z },
    level: 'info',
    message: 'test message',
    timestamp: '2018-07-21T08:59:27.959Z',
    [Symbol(level)]: 'info',
    [Symbol(splat)]:
     [ { data: { a: 1, b: [ 1, 2, 3, 4 ], d: 2018-07-21T08:59:27.958Z } } ] }
  { Error: I'm an error!
      at Object.&lt;anonymous&gt; (/Users/max7z/projects/test/t32__test__winston__pretty-print_symbol/index.js:13:9)
      at Module._compile (internal/modules/cjs/loader.js:689:30)
      at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
      at Module.load (internal/modules/cjs/loader.js:599:32)
      at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
      at Function.Module._load (internal/modules/cjs/loader.js:530:3)
      at Function.Module.runMain (internal/modules/cjs/loader.js:742:12)
      at startup (internal/bootstrap/node.js:266:19)
      at bootstrapNodeJSCore (internal/bootstrap/node.js:596:3)
    level: 'error',
    timestamp: '2018-07-21T08:59:27.962Z',
    [Symbol(level)]: 'error' }
  </code></pre>
  
  <p>I'd like to have almost the same output, but without some fields: [Symbol(level)], [Symbol(splat)].</p>
  
  <p>Is it possible to remove [Symbol] fields from <code>prettyPrint</code> formatter?</p>
  
  <p>Or how can I make my own custom json formatter with error stacktrace like prettyPrint has?</p>