repl: arguments to exec of the form 1_1 are interpreted as 11

[aboodman]

From aboodman/replicant#85

It's because of the argument handling in rep:

https://github.com/aboodman/replicant/blob/master/cmd/rep/main.go#L122

We rely on strconv.ParseFloat from the Go stdlib, and it allows underscores in number literals "for readability". I remember when I wrote this feeling like it would probably come back to bite me 🤷‍♂️🐊.

There is probably a better function somewhere that interprets float literals more strictly.