org.el: Option to get LaTeX preview in the same color as text face
* org.el (org-format-latex-options): Add `auto' to docstring.
(org-format-latex): Get face colors at point and put them inside opt.
(org-create-formula-image-with-dvipng): Fix bug when colors are not
`default'.
(org-create-formula-image-with-imagemagick): Fix bug when handling
"Transparent" bg color.
(org-dvipng-color-format): Same as `org-latex-color-format' for
dvipng-style color specification.
If `auto' is used for the :foreground or :background value in
org-format-latex-options, the the appropriate color is chosen
from the face in which the formula is displayed.