No more positionals after a positional slice or map

[alexflint]

With this PR, go-arg returns an error if you define more positional arguments after a positional slice or map. Such positional arguments cannot be populated by go-arg. This could break programs that define positional arguments after positional slices or maps, but for whatever reason never need them to be populated by go-arg.

#265

[plutonium-239]

Hey, so does this mean even the following use case is unsupported? (Essentially args for a copy program)
Taken from Positional Arguments Example, slightly modified to interchange slice being first:

var args struct {
	Input   []string  `arg:"positional"`
	Output  string    `arg:"positional"`
}

So that the desired command is ./copy input1 input2 input3 output-dir/

I'm currently working on rbcp that already uses go-arg, but I was trying to add a new feature that would require this.

[alexflint]

Hey @plutonium-239 yeah that is correct... unfortunately. The case you describe does not work at present, though it would be reasonably simple to add support for it. It's actually quite amazing that it's never been requested before. I'll add it to my TODOs.