Add render checklist command

[jhnstn]

This adds the first CLI command gbm render checklist

The command outputs the release checklist

try go run main.go render checklist to see the command options

This also updates the render package to accept a map of custom functions that can be called in templates. In this case it's used to render Gutenberg Task blocks

Testing run go run main.go render checklist -v 1.0.0 to generate a scheduled release checklist run go run main.go render checklist -v 1.0.1 to generate a patch release checklist

Try pasting the output into a p2 to verify the post looks correct