Allow complex node paths in `dbschema.json`

```
In `maps`, `*/data.json`, `to_table`:
{
"node": "topics", # Allow more complex paths in form of array
# "node": [], --> The whole file object has to be used in the table
# "node": ["main", "topics"] --> The node data["main"]["topics"] has to be used
"table": "topic"
}
```

Simply allow `to_table:node` to be either a string or an array of strings.
In case of the array the multiple strings specify the path from the root to reach the target to include in the table.

This would allow to use frameworks that natively store namespaced data (i.e. path of two nodes) as for example does [Mavo](http://mavo.io).