Add initial declpac CLI tool implementation

pkg/output/output.go

@@ -0,0 +1,12 @@
+package output
+
+import "fmt"
+
+type Result struct {
+	Installed int
+	Removed   int
+}
+
+func Format(r *Result) string {
+	return fmt.Sprintf("Installed %d packages, removed %d packages", r.Installed, r.Removed)
+}