panic: invalid arguments

I am getting the below error when trying to run `wuzz` with no arguments. I am using ubuntu bash on windows to run this.

```
2017/03/12 21:15:06 invalid argument
panic: invalid argument

goroutine 1 [running]:
panic(0x6fed00, 0xc42000d3e0)
/usr/local/go/src/runtime/panic.go:500 +0x1a1
log.Panicln(0xc420051e60, 0x1, 0x1)
/usr/local/go/src/log/log.go:334 +0xc9
main.main()
/home/achernyak/go/src/github.com/asciimoo/wuzz/wuzz.go:1594 +0xf8c
```

Start at wuzz.go:1594 and reproduce the report by running wuzz with no arguments in Ubuntu Bash on Windows. Trace why the invalid argument is logged and panics; done means the no-argument invocation no longer produces the reported panic.

Written by the indexing model from the issue text.