Add [Vocabulary] annotation

[tpluscode]

As suggested by @asbjornu, it will be good to give users an option to define @vocab using an attribute instead of defining it in the @context:

[Vocabulary("http://schema.org/")]
public class Person
{
}

[tpluscode]

I'm unsure whether it should simply produce

{
   "@vocab": "http://schema.org/"
}

or work like VocabContext<Person>()