Fix up collections

* [x] Reset the collections hash on #reset. (Fixes #2234. Fixes #2233.)
* [x] Fix up `collections` config key:

```yaml
# 1. Allow array
collections:
  - my_collection

# 2. Allow hash
collections:
  my_collection:
    hi: there

# 3. Ask for `write` to write out individual output files for each document
collections:
  my_collection:
    write: true
```