Added possibility to use option groups for dropdown select

Based on #114, added tests and doc.

```javascript
  type: 'Select',
  options: [
    {
      group: "Standart",
      items: ["A", "AAAA", "CNAME", "MX", "SPF", "TXT"]
    },
    {
      group: "Advanced",
      items:  ["NAPTR", "SRV", "PTR", "NS"]
    },
    {
      group: "Special",
      items: ["Redirect"]
    }
  ]
```