Allow users to set the Redis URL for RQDashboard via environmental variables

This is a tiny update to allow users of rq_dashboard to set the information for the redis connection from the environment. 

This came up when I was trying to integrate RQDashboard into my Flask app, with your suggested syntax:

```python
# app.py
RQDashboard(app)
```

The problem was, no matter what I did or how I played with `app.config`, I could never get RQDashboard to recognize my URL.  This was a big problem because I'm running it in Heroku, where not only is my information most likely going to be in environmental variables, but the variable is called `'REDISTOGO_URL'`, not `'REDIS_URL'`, though obviously I can copy the value over if needed.

With this change, a user could get around this issue and specify the URL for RQDashboard without affecting the queue or any other part of the app in any other way, simply by setting an environmental var `RQ_DASHBOARD_REDIS_URL` to the full redis URL.