Audit command-line input parsing for obtuse errors

[hawkw]

Potential follow-on from #31 .

For example, passing a nonsense string to --lengthPercentiles sometimes gets me this:

panic: runtime error: index out of range

goroutine 1 [running]:
github.com/buoyantio/strest-grpc/client/percentiles.ParsePercentiles(0x7fff5fbff873, 0x4, 0x0, 0x0, 0x0)
        /Users/eliza/Code/go/src/github.com/buoyantio/strest-grpc/client/percentiles/percentiles.go:38 +0x2bb
github.com/buoyantio/strest-grpc/client.Config.Run(0x151fbe2, 0xe, 0x0, 0x0, 0x1, 0x1, 0x0, 0x0, 0x2540be400, 0x1519c9e, ...)
        /Users/eliza/Code/go/src/github.com/buoyantio/strest-grpc/client/client.go:388 +0x233
github.com/buoyantio/strest-grpc/cmd.glob..func1(0x17e0a00, 0xc4200496a0, 0x0, 0x1)
        /Users/eliza/Code/go/src/github.com/buoyantio/strest-grpc/cmd/client.go:17 +0x5a
github.com/buoyantio/strest-grpc/vendor/github.com/spf13/cobra.(*Command).execute(0x17e0a00, 0xc420049680, 0x1, 0x1, 0x17e0a00, 0xc420049680)
        /Users/eliza/Code/go/src/github.com/buoyantio/strest-grpc/vendor/github.com/spf13/cobra/command.go:702 +0x2c6
github.com/buoyantio/strest-grpc/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0x17e0c20, 0x17e66d0, 0xc420125f70, 0x1006164)
        /Users/eliza/Code/go/src/github.com/buoyantio/strest-grpc/vendor/github.com/spf13/cobra/command.go:783 +0x30e
github.com/buoyantio/strest-grpc/vendor/github.com/spf13/cobra.(*Command).Execute(0x17e0c20, 0x24, 0xc4201701e0)
        /Users/eliza/Code/go/src/github.com/buoyantio/strest-grpc/vendor/github.com/spf13/cobra/command.go:736 +0x2b
main.main()
        /Users/eliza/Code/go/src/github.com/buoyantio/strest-grpc/main.go:11 +0x31

It would be nice to hook as much of this as possible into Cobra's validation?