feat: add --indent flag to control line indentation in remind output

- Introduces an 'indent' integer flag to the CLI for specifying the
  number of spaces to indent lines in the remind output.
- Updates Task.ToRemind to accept an indent parameter and apply heading
  spaces accordingly.
- Adjusts main.go to pass the indent value from the flag to ToRemind.

internal/parser/parser.go

@@ -26,7 +26,7 @@ type Task struct {
 	Metadata       map[string][]string `json:"metadata"`
 }
 
-func (t Task) ToRemind() string {
+func (t Task) ToRemind(indent int) string {
 	var sb strings.Builder
 	sb.WriteString("REM TODO ")
 	if t.DueDate == nil {
@@ -87,6 +87,8 @@ func (t Task) ToRemind() string {
 		sb.WriteString(" (%b)")
 	}
 
+	headingSpaces := strings.Repeat(" ", indent)
+
 	if len(t.Metadata) > 0 {
 		for k, values := range t.Metadata {
 			if k == "due" {
@@ -95,11 +97,11 @@ func (t Task) ToRemind() string {
 			// uppercase first letter for Remind
 			kUpper := strings.ToUpper(k[:1]) + k[1:]
 			if len(values) == 1 {
-				sb.WriteString(fmt.Sprintf("%%_%s: %%<%s>", kUpper, kUpper))
+				sb.WriteString(fmt.Sprintf("%%_%s%s: %%<%s>", headingSpaces, kUpper, kUpper))
 			} else {
 				for i := range values {
 					kNumbered := fmt.Sprintf("%s%d", kUpper, i+1)
-					sb.WriteString(fmt.Sprintf("%%_%s: %%<%s>", kNumbered, kNumbered))
+					sb.WriteString(fmt.Sprintf("%%_%s%s: %%<%s>", headingSpaces, kNumbered, kNumbered))
 				}
 			}
 		}

main.go

@@ -14,6 +14,7 @@ var (
 	inputFile  string
 	outputFile string
 	debug      bool
+	indent     int
 	version    = "v3.0.0"
 )
 
@@ -59,7 +60,7 @@ func main() {
 			}
 
 			for _, t := range tasks {
-				rem := t.ToRemind()
+				rem := t.ToRemind(indent)
 				if rem == "" {
 					continue
 				}
@@ -71,6 +72,7 @@ func main() {
 	rootCmd.Flags().StringVarP(&inputFile, "input", "i", "", "Input file (default: stdin)")
 	rootCmd.Flags().StringVarP(&outputFile, "output", "o", "", "Output file (default: stdout)")
 	rootCmd.Flags().BoolVar(&debug, "debug", false, "Print intermediate JSON to stderr")
+	rootCmd.Flags().IntVar(&indent, "indent", 0, "Number of spaces to indent lines (from second line onward)")
 	rootCmd.Version = version
 	rootCmd.Flags().BoolP("version", "v", false, "Show version and exit")
 	rootCmd.SetVersionTemplate(fmt.Sprintf("todotxt2remind version %s\n", version))