Place the cmd nunjucks processor behind a flag

[3p3r]

Default should be that it doesn't work / is not available. Add a flag: --allow-insecure. IIRC mu has this exact flag.

We might make it a list like --allow-insecure (allow all) and --allow-insecure cmd,other for a list of things to allow, etc...

This can be done in config.ts with a boolean flag so the preprocessor class does not add its cmd helper to Nunjucks.