Feature: use template instead of StringBuilder

Please see PR #12 for details on this PR; this is basically the same PR.

Note that the original PR was against the develop branch - I think it got closed once that branch was deleted.

To do before this can be pulled in:

 * [x]  review 274da219d85341e4c599feef1a976be877d0d81e and add any new functionality here
 * [x] make sure it can be automatically merged

@andreasohlund 

 * [ ] Please review changes in Fody stuff; not sure if I merged that correctly
 * [ ] I had to [re-add the DueOn sort](https://github.com/openkas/GitHubReleaseNotes/commit/58f29d018184113dc7f41aaca475437027d41a98#diff-0f6089206debabf371506b1f53a75b13R51) - is that OK? It was removed in 274da219d85341e4c599feef1a976be877d0d81e but I don't understand why.
 * [ ] If required, review my earlier comments as part of #12