Suggestion: Default values for args.

It would be nice if there was a way to specify default values for an argument. Then, when you go to fill in the arguments, the default value can already be added to the prompt and you can just hit enter.

Perhaps a token like `=` or `:`, when in the middle of an arg definition?

So a command like this:
`head -n #num=5! #file`
Would already have 5 in the prompt when asking for `#num`.