Feature: use template to render release notes instead of `stringBuilder.Append`

In this PR, I use a Nustache template to render the release notes. This makes it easy for others (myself :smile:) to generate their own release notes (e.g. with a different header / footer).

The current implementation exactly matches the original string builder approach. Before taking this further, I'd like you guys to review this and pull it in.

It's not a big overhaul, although at first sight it might look it is. To get a good picture of what I've done, I suggest you first skim the new [`ReleaseNoteBuider.BuildReleaseNotes`](https://github.com/openkas/GitHubReleaseNotes/blob/feature/use_template/Compiler/ReleaseNotesBuilder.cs#L30) and then read the following commits:

 * bde7c4f this adds the tests so that we're backwards compatible; everything in this commit should probably be removed the moment we move forward using this template based approach
 * 971ccef this shows how I moved stuff to the template
 * 94be5f8 .. bc7f959 only cleanup and moving things within `ReleaseNoteBuilder.cs`

I'd appreciate any feedback.