Add some basic agenda view support.

If `org-ehtml-allow-agenda' is non-nil then org-ehtml supports exporting
agenda views.

- /agenda/(day|week|fortnight|month|year) :: For `org-agenda-list`
- /agenda/todo :: For `org-todo-list`
- /agenda/tags/(todo/)MATCH :: for `org-tags-view`

* src/org-ehtml-server.el (org-ehtml-allow-agenda): New variable.
  (org-ehtml-serve-file): Support agenda views.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>