text/template: add support for optional comment argument in {{end}} tag

[bruth]

Such as {{end "loop"}}. The motivation is to make it easier to read where an action ends when there are many. All endable actions close with a generic {{end}} rather than {{endif}} and {{endwith}}, it makes it difficult to read which {{end}} goes with what action.

[adg]

I don't think this is a substantial improvement over ``{{end}}{{/* loop */}}`.

[bruth]

Thanks, fair enough.