Serialize multiple attributes using Pluck

Nobody has claimed this yet.

Can Pluck be used to deserialize multiple source attributes to a single field and then serialize back to multiple fields? I read in #1315 that the intent of Pluck is to go from flat -> nested -> flat, but I couldn't figure out how to get from the input:

`{'baz': 'blue', 'qux': 'orange'}`

by deserializing to

`{'bar': {'baz': 'blue', 'qux': 'orange'}}`

and then serializing back to the above.

I could get _either_`baz` or `qux` but not both.

I attempted something like

```
class Bar(Schema):
baz = fields.String()
qux = fields.String()

class Foo(Schema):
bar_1 = fields.Pluck(Bar, 'baz', data_key='baz', attribute='bar')
bar_2 = fields.Pluck(Bar, 'qux', data_key='qux', attribute='bar')
```

But this throws the error:

`ValueError: The attribute argument for one or more fields collides with another field's name or attribute argument. Check the following field names and attribute arguments: ['bar']`

Is there something I'm missing, or is this use case not supported by Pluck?

Start by reading fields.Pluck and the behavior discussed in issue #1315, then reproduce the Bar and Foo schemas from this report. Verify whether multiple Pluck fields can share the bar attribute while deserializing and serializing both baz and qux; done means the reported round trip works or the limitation is clearly documented.

Written by the indexing model from the issue text.