Allow to use imagemagick for previewing LaTeX fragements.

Allow to use imagemagick for previewing LaTeX fragements.

* org.el (org-latex-create-formula-image-program): New option
to use either dvipng or imagemagick to convert and preview
LaTeX fragments.
(org-preview-latex-fragment, org-format-latex): Handle the new
option.
(org-create-formula-image-with-dvipng): Rename from
`org-create-formula-image'.
(org-create-formula-image-with-imagemagick): New defun to
handle LaTeX preview with imagemagick.
(org-latex-color, org-latex-color-format): New defuns to
handle color conversions.

* org-latex.el (org-latex-to-pdf-process, org-export-as-pdf):
Allow to use imagemagick to convert LaTeX fragments.

* org-html.el (org-export-html-preprocess): Ditto.

* org-exp.el (org-export-with-LaTeX-fragments): Ditto.

* org.texi (@LaTeX{} fragments): Document imagemagick as an
alternative to dvipng.