Introduce env vars for prefix and suffix body content surrounding Changelog

[tajobe]

Adds BODY_PREFIX and BODY_SUFFIX optional environment variable configurations to enable augmenting release body contents around the Changlog entries.

For example, I want a suffix after my Changlog in each release explaining the various artifacts produced in the release (fat jar vs online download)

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An alternative implementation might be a single variable for eg BODY_FORMAT, which could used something like fmt.Sprintf(rel.BodyFormat, rel.Changelog)