Aggregate: Allow filter defined within aggregate for group

[AndrewSisley]

Part of #28

Is fairly important to be able to do this, particularly for top level aggregates:

query {
    _count(users, filter: {Age: {_gt: 26}})
}

[jsimnz]

just for reference:

query {
    user(filter: {Age: {_gt: 26}}) {
          _count
    }
}

_count equivalent to _count(*) from SQL

[jsimnz]

Part of #28

Is fairly important to be able to do this, particularly for top level aggregates:

query {
    _count(users, filter: {Age: {_gt: 26}})
}

quick edit:

query {
    _count(collection: users, filter: {Age: {_gt: 26}})
}

[AndrewSisley]

query {
  author {
    name
    numberOfBooks: _count(field: books, filter:  {rating: {_gt: 4}})
  }
}