Allow list fields to be grouped with headings

[JedWatson]

Like this:

User.add({
    name: { type: Types.Name, required: true, index: true },
    email: { type: Types.Email, initial: true, required: true, index: true },
    phone: { type: String, width: 'short' },
    password: { type: Types.Password, initial: true, required: true }
}, 'Permissions', {
    isAdmin: { type: Boolean, label: 'Can access Keystone' }
};

Any strings passed as arguments to the List.add method should indicate a heading.