add multiple select support

```
multiple_select_field: {
    type: 'string',
    title: 'test multiple',
    multiple: true,
    default: ['a','b'],
    enum: ['a','b','c','d','e']
}
```

I implemented a small ability to multi select field. maybe I did not properly use the default value? I'm putting out there saved data from db json... but for me it works. I think that features may be interesting for someone, how do you think?