Refactor into mixins + create search mixin pkg

[matteodem]

Allow to pass mixins (or some other naming) to extend graphqlize resolvers and type defs. Also refactor CRUD to be mixins (one for create, one for read etc).

Make the search mixin a separate package and provide some simple default mongo search query.

import { createCollectionSchema } from 'meteor/easy:graphqlizer'
import { searchMixin } from 'meteor/graphqlizer:search'

createCollectionSchema({ 
  type: 'Alien',
  collection: AlienCollection,
  schema: AlienSimpleSchema,
  mixins: [searchMixin],
})

const createMixin = {
  Mutation: {
  },
  typeDefs: `
extend type Mutation {
  ...
}
`
}

Verify if the config structure still makes sense with mixins. Probably deprecate crud config