Introduce multiple context feature to @Json

[etsuo]

Models may have multiple sources and they may need to be marshalled to multiple destinations, of which each may have different field names for the same property.

@Model()
export class SomeClass extends SapiModelMixin() {

    @Db('fn')
    @Json('fName')
    @Json({field: 'first_name', context: 'sisense'})
    @Json('f', 'source2')
    firstName: string;
}

const someClass1 = SomeClass.fromJson({'firstName': 'John'}); // depending on #121
const someClass2 = SomeClass.fromJson({'fName': 'John'});
const someClass3 = SomeClass.fromJson({'first_name': 'John'}, 'sisense');
const someClass4 = SomeClass.fromJson({'f': 'John'}, 'source2');

someClass1.toJson();
// { "fName": "John" }

someClass2.toJson('sisense');
// { "first_name" : "John" }

someClass3.toJson('source2');
// { "f" : "John" }