Add method to configure system process attributes

[NeedsSoySauce]

Adds a method to set system process attributes on the process used to run exiftool.

For example, on windows to run exiftool without creating a window:

const CreateNoWindow = 0x08000000
var sysProcAttr = syscall.SysProcAttr{
	HideWindow:    true,
	CreationFlags: CreateNoWindow,
}
et, err := exiftool.NewExiftool(exiftool.SystemProcessAttributes(&sysProcAttr))

I'm not exactly sure what a good way is to test this? An option I considered was replacing exec.Command with a fake so that SysProcAttr can be inspected when the command is actually run. If that sounds okay let me know and I can do it.