Allow scanning to print to stdout

[tonglil]

Instead of forcing a destination file, print the scan result to stdout if a file is not provided.

[tonglil]

Can be done, but is it desirable?

• Update scan.go to accept no args or a --stdout flag.
• Refactor scan.go to create the Options object via a function that checks for an "" Filename and sets a private var to print to stdout, maybe like:

func NewOptions(dryrun, repo pflag.Flag, file string) Options {
		opt := &types.Options{
			DryRun:   dryrun.Changed,
			Repo:     repo.Value.String(),
			Filename: file,
		}

	if file == "" {
		opt.Stdout = true
	}
}

• Skip file operations in reader.go if flag is set.
• Write to stdout instead of file in reader.go.

Could be better served by big refactoring of Options to hold a file open, than just the filename. Or change config.Write to satisfy some interface.

[tonglil]

But the question is why do this? Dry run is already pretty decent and scanning into a file is just a matter of git reset to revert the scan results.