configuration examples

```yaml
MONGODB_URI: mongodb://localhost:27017/dbname?replicaSet=replset

# or
MONGODB_URI: "/tmp/mongodb-27017.sock"

# or
# ---- with aliases
MONGODB:
  default:
    db: dbname
    host: localhost
    port: 27017
    username: user
    password: pass
    replicaSet: replset
    replicaSetOptions:
      readPreference: secondaryPreferred # must be one of ``primary``, ``primaryPreferred``, ``secondary``, ``secondaryPreferred``, or ``nearest``
      maxStalenessSeconds: -1
    options:
      appname: some_app_name
      heartbeatFrequencyMS: 10000
      # .... refer to MongoClient documentation from pymongo website
  "<alias>":
    db: dbname
    host: localhost
    # .... same as default

# --- without and many
MONGODB:
  - db: dbname
    host: localhost
    port: 27017
    # ....
  - db: dbname2
    host: localhost
    port: 27018
    # ....
  - db: dbname3
    host: localhost
    port: 27018
    # ....
```

@TODO doc, tests