Support connection string

[buo]

It would be very nice if I could connect to MongoDB using one connection string ruling them all instead of passing user, pass, host, port, and database.

[orion110217]

I think it's much easier to configure the current setup between environments.

[buo]

Or, could we support it as optional? I found it's a bit troublesome when I deploy my app on PaaS like Heroku because all MongoDB add-ons gave me just one environment variable having the connection string, so I had to parse them.

And what about connecting to multiple hosts in a replica set?

[anvlkv]

solved here https://github.com/Jackong/koa-mongoose/pull/11