[csvwlib] properties at table level

in csvw you typically register some metadata elements at table level

it is also possible to describe multiple tables in a single csv

this relates to #232, suggesting to move the attributes element one level deeper

```yaml
content-info:
- featuretypes:
- name: trees
url: https://www.wikidata.org/wiki/Q10884
attributes:
- name: length
- name: type
- name: plants
url: https://www.wikidata.org/wiki/Q756
attributes:
- name: length
- name: type
```