Configurable database connection strings.

[ccampo133]

Currently each database connection string is hardcoded in the respective sql.Repository implementation. This is a bit inflexible for users who have require more complicated database connections, such as configuring TLS or proxy settings. This is mitigated at least for Postgres since the driver allows for environment variables, although we're not so lucky for other

One option is to make the entire connection string as an input parameter. Another option would be to accept a set of key/values which we can append to the connection string. I'm open to suggestions here.