Explicit parameter generation syntax in templates

- Add "generate" field to specify generator to be used
- Make "value" field the input value for the generator
- Remove parameter "type" field, it's always "string"
Ok, then I need to distinguish `Value` (the result value) and `From` (the input for the generator), right?

```
{
"name": "Param",
"generate": "expression",
"from": "[a-z]{10}",
/* "value":  <- to be generated */
}
```