Better handling of export options.

Better handling of export options.

This patch improves the internal handling of the large number of
export options that Org-mode has.  In particular, it now defines a
single constant, `org-export-plist-vars' that contains the full list
of options, and how these can be accessed through variables, and
through the #+OPTIONS line.  Before the patch, there used to be a
separate list or the OPTIONS handling, which has gotten out of sync.
This now no longer can happen.

Also, this patch adds two new variables, `org-export-with-priority',
and `org-export-with-todo-keywords', which can be used to turn the
export of TODO keywords and priority cookies on and off, so that these
can simply disappear and not be shown in the exported file.  In fact,
the default for the priority cookie is to remove it.