Don't show agendas by default

Because this behavior may be unexpected to those simply using org-ehtml
to display editable files, because it could overlap file paths, and
because it may expose more information about the running Emacs process
than the user anticipates.

* src/org-ehtml-server.el (org-ehtml-allow-agenda): Default to nil.
README: Add entry for the Agenda View.

* README (Agenda View): New entry.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
Agenda View: Provide hints on error page.

* src/org-ehtml-server.el (org-ehtml-serve-file): Provide hints if
  agenda command is unknown.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>