function defaultEmphasis(opt, key, subOpts) {
  // Caution: performance sensitive.
  if (opt) {
    opt[key] = opt[key] || {};
    opt.emphasis = opt.emphasis || {};
    opt.emphasis[key] = opt.emphasis[key] || {}; // Default emphasis option from normal

    for (var i = 0, len = subOpts.length; i < len; i++) {
      var subOptName = subOpts[i];

      if (!opt.emphasis[key].hasOwnProperty(subOptName) && opt[key].hasOwnProperty(subOptName)) {
        opt.emphasis[key][subOptName] = opt[key][subOptName];
      }
    }
  }
}
Sync default option between normal and emphasis like `position` and `show`
In case some one will write code like
    label: {
         show: false,
         position: 'outside',
         fontSize: 18
    },
    emphasis: {
         label: { show: true }
    }