Implement escaping in src (org only) and example blocks

inside src example blocks lines starting with `\s*,` are escaped - i.e. org
mode will not try to parse them as e.g. a headline. We don't want to render the
escape commata to html so let's take them out - and put them back in before
rendering to org. Doing it this way allows us to render them correctly even
when the input did not include them.

see https://orgmode.org/manual/Literal-Examples.html#Literal-Examples

org/org_writer.go

@@ -2,6 +2,7 @@ package org
 
 import (
 	"fmt"
+	"regexp"
 	"strings"
 	"unicode"
 	"unicode/utf8"
@@ -16,6 +17,8 @@ type OrgWriter struct {
 	indent string
 }
 
+var exampleBlockUnescapeRegexp = regexp.MustCompile(`(^|\n)([ \t]*)(\*|,\*|#\+|,#\+)`)
+
 var emphasisOrgBorders = map[string][]string{
 	"_":   []string{"_", "_"},
 	"*":   []string{"*", "*"},
@@ -90,7 +93,11 @@ func (w *OrgWriter) WriteBlock(b Block) {
 	if isRawTextBlock(b.Name) {
 		w.WriteString(w.indent)
 	}
-	WriteNodes(w, b.Children...)
+	content := w.WriteNodesAsString(b.Children...)
+	if b.Name == "EXAMPLE" || (b.Name == "SRC" && len(b.Parameters) >= 1 && b.Parameters[0] == "org") {
+		content = exampleBlockUnescapeRegexp.ReplaceAllString(content, "$1$2,$3")
+	}
+	w.WriteString(content)
 	if !isRawTextBlock(b.Name) {
 		w.WriteString(w.indent)
 	}