New variable to pass options to the TeX processor.

* tex.el (TeX-command-extra-options): New customizable variable.
(TeX-expand-list): New `%(extraopts)' expander.
(TeX-command-list): Use `%(extraopts)'.

* doc/auctex.texi (Processor Options): Document
`TeX-command-extra-options'.

* doc/changes.texi: Document it.