Allow setting the init command on the connection.

When reconnect is enabled, it is handled within libmysqlclient.

In such case, if a connection sets any session variables after connecting,
they will be lost after the reconnection.
Update README with information about the init_command option
Moving the init_command section in README
Merge pull request #501 from marcinbunsch/init-command

Allow setting the init command on the connection.