Propose: Adding a '--enable-failover' option to 'pg_createsubscriber'

Hi

A failover option has been added to the CREATE SUBSCRITION command, but this functionality isn't easily accessible using the pg_createsubscriber tool.

Subscriptions created using pg_createsubscriber must be configured for failover via an alter operation.

To address this issue, we've added a simple --enable-failover option to the pg_createsubscriber tool.

This patch is simple. It doesn't handle exceptions or provide any TAP test code.

Please review this and we hope the development team will refine it further.

ioseph