org-table.el: Add defcustoms for LaTeX export of table rules :tstart, :hline and :tend
* org-table.el (orgtbl-to-generic): Add check for :skipheadrule.
When present, the :hline following the head will be skipped. This is
necessary to avoid doubling of horizontal rules in LaTeX longtable
environments and consequent width problems.
* org-latex.el (org-export-latex-tables-tstart)
(org-export-latex-tables-hline)
(org-export-latex-tables-tend): New options.
(org-export-latex-tables): Use the new options.