Allow nilable nested params

[vladfaust]

Should choose one of these variants. Please vote with according emoji.

A 👍

# Nilable
type user? do
  type id : Int32
end

# Non-nilable
type user do
  type id : Int32
end

B 👎

# Nilable
type user : Object | Nil do
  type id : Int32
end

# Non-nilable
type user : Object do
  type id : Int32
end

# Non-nilable
type user do
  type id : Int32
end

C 😕

# Nilable
type user, nilable: true do
  type id : Int32
end

# Non-nilable
type user, nilable: false do
  type id : Int32
end

# Non-nilable
type user do
  type id : Int32
end