Allow user to specify a config file with a different structure

[alexbrazier]

Allow the user to specify where in the config file the db options are.

e.g.

{
    "database": {
        "production": {
            "host": "localhost",
            "db: "test"
        }
    }
}

To run: mongo-migrate -runmm -cfg config.json -dbn "database.production" <command>