Fold example and source blocks within a new native environment.

Fold example and source blocks within a new native environment.

* lisp/org-exp.el (org-native-text): New text property to mark
blocks of text that are in native format.
(org-export-select-backend-specific-text): Mark
text within #+backend, #+begin_backend...#+end_backend with
`org-native-text' text properties.
(org-export-do-format-source-code-or-example): Mark text
within #+begin_html and #+end_html with `org-native-text'
property.

* lisp/org-html.el (org-do-export): Handle insertion of native
text within a native environment.
(org-html-format-environment): Handle native environment.

* lisp/org-odt.el
(org-export-odt-format-source-code-or-example): Mark text
within #+begin_odt...#+end_odt with `org-native-text'
property.
(org-odt-begin-environment, org-odt-end-environment): Handle
native environment.