Allow couchbase-cli to disable/enable setting-alert without disturbing existing settings

Currently "couchbase-cli setting-alert" will replace all settings with only those specified on the command line. This makes it difficult to temporarily disable then re-enable alerts.

In order to accomplish a disable then enable, the existing settings would need to be captured, stored and specified on the command line when re-enabling. It would be helpful to add an option to couchbase-cli which could disable or enable while retaining all other current settings.

Couchbase Web Console accomplishes this by first getting the settings through the API to fill in the HTML form. If "Enable email alerts" is unchecked then saved, all the existing settings are POSTed so that alerts can later be enabled with the prevoius settings still intact.