Create codefmt usage screencast for README.md

Lots of vim plugins have fancy screencasts (animated GIFs) in the README.md showcasing plugin features. The codefmt plugin has several useful features that would make a nice screencast.

We should create a screencast and embed it in our README.md. If we could also include a keyboard monitor in the screencast, that would help show off all the handy mappings.

首先查看 README.md 以及 codefmt 插件文档中记录的功能和映射，以选择 screencast 应该演示的内容。使用键盘监视器创建 screencast，将其嵌入 README.md，并验证结果是否清晰展示了该插件的实用功能。

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