support recurse option

At the moment envconsul doesn't support returning json like object when folder is queried. Instead it returns None.
For instance we have following k/v pairs:
```
services/scheduler/TASKS/mm_tick/expiration: 9
services/scheduler/TASKS/mm_tick/interval_sec: 10
```
I'd like to have an option to query `services/scheduler/TASKS` and get something like:
```
[{"mm_tick": {"expiration": 9, "interval_sec": 10}}]
```