Support partial results

[knazarov]

Add support for getting only a subset of fields. Like this:

crud.get('customers', 1, {fields={'id', 'name'}})
---
- metadata:
  - {'name': 'id', 'type': 'unsigned'}
  - {'name': 'name', 'type': 'string'}
  rows:
  - [1, 'Elizabeth']
...

[knazarov]

Note: it is possible that we'd want to keep bucket_id and primary key always in the result, no matter if they were specified or not. Let's think about it.

[dokshina]

Closed by #116, #117