Use default value - HOST and PORT

Hi,

Thanks for the extension. Would you mind to use default values for HOST and PORT, if env vars are not set?

- MYSQL_HOST: The hostname or IP address of the MySQL server.
- MYSQL_PORT: The port number of the MySQL server.

"localhost" could be the default value for host.
And port 3306 is very common.

Thanks for consideration.

Cheers