Improve footnotes handling in exporters
* lisp/org-exp.el (org-export-footnotes-markers,
org-export-footnotes-data): new variables.
(org-export-preprocess-string): use a more explicit argument.
* lisp/org-html.el (org-export-as-html): initialize new variables.
* lisp/org-docbook.el (org-export-as-docbook): initialize new variables.
* lisp/org-latex.el (org-export-latex-footmark-seen): new variable.
(org-export-as-latex): initialize new variables.
(org-export-latex-preprocess): allow to export two or more footnotes
in a row. Also permit to have footnotes refering to previously
defined footnotes.
* lisp/org-ascii.el (org-export-as-ascii): feed org-footnote-normalize
with data so it can normalize footnotes before first headline, or
footnotes with their definition outside exported region.