Add \ to emphasis post chars to allow immediate explicit line break

Post chars are defined in (nth 1 org-emphasis-regexp-components) in emacs org.
When I initially adapted the list of chars for go, I failed to check how it's
actually used (further down in org.el):
  (string-match (concat "[" (nth 1 erc) "\n]") (char-to-string (char-after (point))))

Due to the surrounding [] the `\\` in
  '("-[:space:]('\"{" "-[:space:].,:!?;'\")}\\[" "[:space:]" "." 1)

is actually a literal backslash, not an escape of the opening bracket I
guess. I'm not in the mood for thinking any harder about this, so let's hope
this is right. yolo.

org/inline.go

@@ -394,7 +394,7 @@ func isValidPreChar(r rune) bool {
 }
 
 func isValidPostChar(r rune) bool {
-	return r == utf8.RuneError || unicode.IsSpace(r) || strings.ContainsRune(`-.,:!?;'")}[`, r)
+	return r == utf8.RuneError || unicode.IsSpace(r) || strings.ContainsRune(`-.,:!?;'")}[\`, r)
 }
 
 func isValidBorderChar(r rune) bool { return !unicode.IsSpace(r) }