Add @optional( <name> ) annotation for optional fields

[VeaaC]

a common pattern used in flatdata is too use a fixed value to indicate a missing field, e.g.

const u32 INVALID_INDEX = 0xFFFFFFFF;
struct MyStruct {
    @const( INVALID_INDEX )
     index : u32;
}

I propose making this a use case that we should support properly:

• add @optional( <const_name> ) annotation
• generate Option<T>/boost::optional/whatever getters/setters
• Allow fast access to raw data via special getter (e.g. field_name_raw()?)

[VeaaC]

Duplicates #80