org.el: Implement user-defined table sorting

org.el: Implement user-defined table sorting

* lisp/org.el (org-do-sort): Implement the ?f and ?F sorting options
  to allow user-defined table sorting.  Update the DOC string.

* lisp/org-table (org-table-sort-lines): Add the GETKEY-FUNC and
  COMPARE-FUNC optional parameters and pass them to the call to
  `org-do-sort'.  Update the DOC string.

* doc/org.texi (org-table-sort-lines): Update documentation to reflect
  the addition of the ?f and ?F options.

This patch implements user-defined extraction and comparison functions
for table sorting.  Thanks to Nicolas Goaziou for helpful suggestions.

This patch was discussed on the Org Mode mailing list:
http://article.gmane.org/gmane.emacs.orgmode/93334