FlaskDB with existing proxy
Argh. This can be closed. After posting, I wondered why I was using FlaskDB in the first place. I updated my code to just connect/disconnect before/after app requests. No FlaskDB needed.

``` python
@app.before_request
def connect_proxy():
    initialize_proxy()
    proxy.connect()

@app.teardown_request
def close_proxy(exc):
    if not proxy.is_closed():
        proxy.close()
```

I have an existing set of models that I'm trying to integrate into a new Flask app. For all these models, I'm using a Proxy object as the database.

It would be great if FlaskDB accepted a Proxy instance as the `database` argument, but it seems like it only works with a Database instance.

Here's a minimal reproducible example:

```python
from flask import Flask
from peewee import Proxy
from playhouse.flask_utils import FlaskDB

app = Flask(__name__)

proxy = Proxy()
proxy.initialize('sqlite:///:memory:')

db = FlaskDB(app=app, database=proxy)

app.run()
```

Results in...

```
File "/opt/.virtualenvs/flask/lib/python3.5/site-packages/peewee.py", line 421, in __getattr__
    return getattr(self.obj, attr)
AttributeError: 'str' object has no attribute 'database'
```

Seems like the chance would need to be in the [_load_database](https://github.com/coleifer/peewee/blob/master/playhouse/flask_utils.py#L103) method. Or is there another/better way to use an existing proxy with FlaskDB?