highlight: support highlighting lines in the default writer

The hl_lines key is specifically the same as Hugo's key.

org/util.go

@@ -1,5 +1,10 @@
 package org
 
+import (
+	"strconv"
+	"strings"
+)
+
 func isSecondBlankLine(d *Document, i int) bool {
 	if i-1 <= 0 {
 		return false
@@ -17,3 +22,49 @@ func isImageOrVideoLink(n Node) bool {
 	}
 	return false
 }
+
+// Parse ranges like this:
+// "3-5" -> [[3, 5]]
+// "3 8-10" -> [[3, 3], [8, 10]]
+// "3  5 6" -> [[3, 3], [5, 5], [6, 6]]
+//
+// This is Hugo's hlLinesToRanges with "startLine" removed and errors
+// ignored.
+func ParseRanges(s string) [][2]int {
+	var ranges [][2]int
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return ranges
+	}
+	fields := strings.Split(s, " ")
+	for _, field := range fields {
+		field = strings.TrimSpace(field)
+		if field == "" {
+			continue
+		}
+		numbers := strings.Split(field, "-")
+		var r [2]int
+		if len(numbers) > 1 {
+			first, err := strconv.Atoi(numbers[0])
+			if err != nil {
+				return ranges
+			}
+			second, err := strconv.Atoi(numbers[1])
+			if err != nil {
+				return ranges
+			}
+			r[0] = first
+			r[1] = second
+		} else {
+			first, err := strconv.Atoi(numbers[0])
+			if err != nil {
+				return ranges
+			}
+			r[0] = first
+			r[1] = first
+		}
+
+		ranges = append(ranges, r)
+	}
+	return ranges
+}