ox-latex: Support tikz images, :width, and :height

ox-latex: Support tikz images, :width, and :height

* ox-latex.el:
(org-latex-image-default-option): Change default value to ""
(org-latex-image-default-width)
(org-latex-image-default-height): Add variables
(org-latex-inline-image-rules): Make .tikz files as exportable with
latex
(org-latex--inline-image): Support tikz images.  Also support separate
:width and :height parameters for images.

* ob-R.el (org-babel-R-construct-graphics-device-call): Change file
extension of tikz graphics files to .tikz

Tikz graphics should be exported to LaTeX by \include, not as a link.
This commit changes the file extension used for tikz graphics from .tex
to .tikz, and inserts code for including such images.  The :options for
tikz graphics are passed as an optional argument to a tikzpicture
environment.

Also provide :width and :height ATTR_LATEX entries for images.  For tikz
graphics, these are implemented with \resizebox; for other image types
they are inserted in the optional arguments to \includegraphics.