fix panic resolver's configuration file does not have read permission

[zbindenren]

Without this fix you get the following panic when a resolver's file is not readable:

command --help
panic: ~/.config/command/config.yaml: open /home/user/.config/command/config.yaml: permission denied

goroutine 1 [running]:
github.com/alecthomas/kong.Parse(0xacf6c0, 0xc0000b8900, 0xc0000b0300, 0x8, 0x8, 0x22)
        /home/rz/golang/pkg/mod/github.com/alecthomas/kong@v0.2.16/global.go:11 +0x145
main.main()

The easiest way for read permission is via os.Open

[alecthomas]

Thanks!