Allow generic REDIS_PROVIDER env var

Setting REDIS_PROVIDER to the name of the env var that has your redis
URL will use that URL. So for example, if your provider is OpenRedis,
you'd want:

    REDIS_PROVIDER=OPENREDIS_URL
    OPENREDIS_URL=redis://...

Same for any other provider. Closes #51.

cc @atmos @ajsharma