Better validation of stages

[na--]

Currently k6 doesn't properly validate the values in target and duration values in stages. Negative duration and target are allowed, with undefined behavior, when they should be forbidden. Also, a single stage with a duration of 0 (stages: [ { duration: 0, target: 5 } ],) causes a panic:

panic: strings: negative Repeat count-------------------------------] starting

goroutine 1 [running]:
strings.Repeat(0xd141c4, 0x1, 0x800000000000003a, 0x0, 0xc422ebc3a8)
	/usr/lib/golang/src/strings/strings.go:538 +0x1c7
github.com/loadimpact/k6/ui.ProgressBar.String(0x3c, 0x7ff0000000000000, 0xc422ebbf68, 0xc422ebbf58, 0xe8ff01, 0xc425d36980)
	/home/user/go/src/github.com/loadimpact/k6/ui/progress_bar.go:57 +0x408
github.com/loadimpact/k6/cmd.glob..func11(0x14dc500, 0xc420250520, 0x1, 0x1, 0x0, 0x0)
	/home/user/go/src/github.com/loadimpact/k6/cmd/run.go:380 +0x25ba
github.com/loadimpact/k6/vendor/github.com/spf13/cobra.(*Command).execute(0x14dc500, 0xc4202504f0, 0x1, 0x1, 0x14dc500, 0xc4202504f0)
	/home/user/go/src/github.com/loadimpact/k6/vendor/github.com/spf13/cobra/command.go:762 +0x468
github.com/loadimpact/k6/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0x14dc2a0, 0x52, 0xc4204f81a0, 0xc4203a4000)
	/home/user/go/src/github.com/loadimpact/k6/vendor/github.com/spf13/cobra/command.go:852 +0x30a
github.com/loadimpact/k6/vendor/github.com/spf13/cobra.(*Command).Execute(0x14dc2a0, 0xc420283f78, 0x405c6c)
	/home/user/go/src/github.com/loadimpact/k6/vendor/github.com/spf13/cobra/command.go:800 +0x2b
github.com/loadimpact/k6/cmd.Execute()
	/home/user/go/src/github.com/loadimpact/k6/cmd/root.go:87 +0x31
main.main()
	/home/user/go/src/github.com/loadimpact/k6/main.go:28 +0x20

It's also unclear how well 0-duration stages (i.e. "suddenly jump up or down from X to Y active VUs") are supported - they should be, we just have to verify it and probably write some tests.

[na--]

This also happens when a script is ran with --vus 1 --iterations 0 --duration 0 ...

[na--]

Closing this, since the just-merged #1007 implements much better validation and fixes the panic