New variable to set the initial export scope.
* org-exp.el (org-export-initial-scope): new custom variable.
(org-export): Use this new variable. If there is an active
region, tell it when prompting the user for an export command.
Also change the way the function handles selection of buffer
and subtree export.
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Now `1' switches between subtree/buffer export until the user
select an export command. Also, when there is an active region,
modify the message so that the user is aware he will only export
the surrounding subtree.