ox-latex: Introduce LATEX_HEADER_EXTRA keyword
* lisp/ox-latex.el (latex): Introduce new buffer keyword.
(org-latex-template): Use new keyword.
* lisp/ox-beamer.el (org-beamer-template): Use new keyword.
* lisp/org.el (org-create-formula--latex-header): Use new keyword.
* contrib/lisp/ox-koma-letter.el (org-koma-letter-template): Use new
keyword.
Previewing of LaTeX snippets takes account of LATEX_HEADER keywords.
LATEX_HEADER_EXTRA keyword allows to specify some header lines that
will not be used for building snippets.