add handling of custom labels

[danielmischler]

Possible use cases:

• enums:

type Address {
  prefix: Prefix
  firstName: String
  lastName: String
}
enum Prefix {
  MR
  MRS
  MS
}

• state:

MERGE (a:Address {...}) ON CREATE SET a:NEW ON MATCH SET a:UPDATED RETURN ...

type Order {
  isPending: ...
  isCancelled: ...
}

• custom:

type Person {
  tagCloud: [String]
}