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.

首先检查 npm start 入口点和所引用的 bin/gren.js 路径，因为 issue 报告称源代码设置无法运行。跟踪 release notes 和本地 changelogs 的生成方式，然后验证 dry run 能够预览这两个输出，而不会覆盖现有的 release。

由索引模型根据 Issue 内容生成。