feat: add output flag with Destination interface.

Added -o flag to write results to files using a new Destination interface.

Changes:
- Add Destination interface with StdoutDestination and FileDestination implementations.
- Add -o flag to CLI parser for output file specification.
- Update Config to use Destination instead of direct output handling.
- Refactor main to use Destination.Write() for result output.

internal/config/destination.go

@@ -0,0 +1,27 @@
+package config
+
+import (
+	"fmt"
+	"os"
+)
+
+// A method of writing output to the user.
+type Destination interface {
+	// Write data to this destination.
+	Write(data string) error
+}
+
+// A destination writing to stdout.
+type StdoutDestination struct{}
+
+func (d StdoutDestination) Write(data string) error {
+	fmt.Println(data)
+	return nil
+}
+
+// A destination writing to a file.
+type FileDestination struct{ Path string }
+
+func (d FileDestination) Write(data string) error {
+	return os.WriteFile(d.Path, []byte(data+"\n"), 0644)
+}