Field custom transformer support

[dianlight]

It can be useful to have custom function that transforms the field value after deserialize or before serialize.
Use the transformers option to specify read or/and write function:

class Type2Element extends BaseElement {
    @Field(version, { transformers: {read: v => v/100, write: v => v * 100 })
    version : number;
}

In this example version value 1.20 is the integer 120 on the stream.