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" {
@@ -99,7 +101,7 @@ func (t Task) ToRemind() string {
 			} 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))
 				}
 			}
 		}