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](https://github.com/machinezone/configmapsecrets) and use only SealedSecrets instead!

I do have one issue with configuration files that contain golang-based templates ([mainly alertmanager configuration like this one](https://github.com/thaum-xyz/ankhmorpork/blob/master/apps/monitoring/manifests/alertmanager/secret.yaml)). 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

Start by locating the Go template-rendering entry point used for injecting secrets into larger configuration files, then compare its behavior with the standard library's text/template Delims option. Done means users can configure non-default delimiters for this rendering path, while the issue's alertmanager example no longer conflicts with Go templates.

Written by the indexing model from the issue text.