Add support for ": example" elements

org/example.go

@@ -0,0 +1,26 @@
+package org
+
+import (
+	"regexp"
+)
+
+type Example struct {
+	Children []Node
+}
+
+var exampleLineRegexp = regexp.MustCompile(`^(\s*): (.*)`)
+
+func lexExample(line string) (token, bool) {
+	if m := exampleLineRegexp.FindStringSubmatch(line); m != nil {
+		return token{"example", len(m[1]), m[2], m}, true
+	}
+	return nilToken, false
+}
+
+func (d *Document) parseExample(i int, parentStop stopFn) (int, Node) {
+	example, start := Example{}, i
+	for ; !parentStop(d, i) && d.tokens[i].kind == "example"; i++ {
+		example.Children = append(example.Children, Text{d.tokens[i].content})
+	}
+	return i - start, example
+}