feat template: Add additional template functions

This adds the Go template functions from [sprig][] as well as a custom `json`
function that can interpolate any data as a JSON object - very useful for adding
arrays of data in JSON format into a variable:

```
certificateDomains:
  - oslo.pub
  - tazj.in

annotations:
  acme/certificate: {{ .certificateDomains | json }}

annotations:
  acme/certificate: ["oslo.pub", "tazj.in"]
```

[sprig]: https://godoc.org/github.com/Masterminds/sprig

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