Support JSON schema

Currently confeta only supports javascript object as a config.
In order to support JSON we need to add handling for types specified as strings

Currently:
```
url: {
  type: types.string,
  description: 'Database url',
  required: true
}
```

Need support for:
```
"url": {
  "type": "string",
  "description": "Database url",
  "required": true
}
```
