dry run

I'm commenting on an issue that was already raised 3 years ago in #167, the scope of mine explains more and requests further things.

I'm want to see the release notes that green generates before committing. It is telling me that a release already exists, which is absolutely true, but I want to see what gren generates.

I can't do this because it's going to instantly overwrite the release notes for a release I've already pushed if I use the overwrite switch, this 100% not what I want to happen, I want to compare what I have against what green would generate.

The same is true of changelogs, although because green only creates them locally, it's not so much of an issue.

Please consider adding a --dry-run option.

I have been looking at the code, but I'm not really familiar with node and I can't get it to run from source, if I use npm start it complains that "./bin/gren.js" does not exist, which is absolutely true. There's no instructions that I could find in this repo on how to run it/build it/install it from source.

Start by inspecting the npm start entry point and the referenced bin/gren.js path, since the issue reports that the source setup cannot run. Trace how release notes and local changelogs are generated, then verify that a dry run previews both outputs without overwriting an existing release.】【。

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