More flexible serialization

[GDXRepo]

https://github.com/undabot/izzyparser-ios/issues/6

[GDXRepo]

Just an example. Allows using nil values (they just will be skipped), and allows to customize multiple attributes and/or relationships in one calls sequence.

Usage may look like this (inside a class with customAttri and width optional non-objc properties, for example, of type Double?):

return IzzyBuilder(resource: self)
    .attribute("customAttri", value: customAttri)
    .attribute("width", value: width)
    .serialize()