Allow extra parameters for MySQL DB

It’s difficult and non-intuitive to add mysql DB parameters to the mysql db datasource type.

A user got this working by setting:

```
"dataSource": "user:password@tcp(127.0.0.1:3306)/smallstepdevmiclain?tls=true&",
"database": "smallstepdevmiclain"
```

but we should really have a separate arg for a parameter string.
