Hook to manage relationships

[daffl]

Follow up from https://github.com/feathersjs-ecosystem/feathers-sequelize/issues/211. Managing relationships with Sequelize is quite strange. For example it doesn't allow updating nested entries and creating nested relationship does not include them in the result (you have to re-fetch). There is an opportunity to add hooks that can help with that:

• Go through each associated entry
• Check if it has an id
  • If it does check if it has the association
  • If not, add it
• If it doesn't, leave it