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.

Start by reviewing README.md and the codefmt plugin's documented features and mappings to choose what the screencast should demonstrate. Create the screencast with a keyboard monitor, embed it in README.md, and verify that the result clearly showcases the plugin's useful features.

Written by the indexing model from the issue text.