default.latex: Redefine includegraphics to allow custom options.

Thanks to Bernhard Weichel.
Now, literal \includegraphics commands with square-bracketed options
will work properly.  When no options are given, a default option
limits images to the text width.
default.latex:  Use \ScaleIfNeeded w/ nicer def instead of \maxwidth.
default.beamer:  Added comment to graphics section.