Get the level string for a log, e.g "INF", "DBG" etc.
Also adds colour to the when enabled.
@param  {Object} opts
function getLevelStr (opts) {
  var lvlStr = _.invert(LEVELS)[opts.level]
    , useColours = opts.colourise || true;

  if (useColours) {
    switch (opts.level) {
      case LEVELS.DBG:
        lvlStr = colours.magenta(lvlStr);
        break;
      case LEVELS.INF:
        lvlStr = colours.green(lvlStr);
        break;
      case LEVELS.WRN:
        lvlStr = colours.yellow(lvlStr);
        break;
      case LEVELS.ERR:
        lvlStr = colours.red(lvlStr);
        break;
    }
  }

  return lvlStr;
}