Configurable template delimeters

First of all, thank you so much for this project and for adding https://github.com/bitnami-labs/sealed-secrets/pull/580 in the latest release! This allowed me to remove my dependency on ConfigMapSecret and use only SealedSecrets instead!

I do have one issue with configuration files that contain golang-based templates (mainly alertmanager configuration like this one). Would it be possible to either use non-default delimiters for injecting secret into larger config? Or maybe it would be possible to allow configuring delimiters and allowing something different than the default {{ and }}?

Just to note, there is an option for this in the standard library: https://golang.org/pkg/text/template/#Template.Delims