Latex filter: Added support for generating SVG images

It is now possible to choose to generate SVG images instead of PNG
images in the Latex filter. SVG is a scalable format which look good
both in print and on the web. This makes it easier to separate content
from presentation.
Renamed latex2png.py to latex2img.py

The latex2png.py script have now been renamed to latex2img.py since it
can now generate both PNG and SVG image formats.
Latex filter improvments

The Latex filter SVG rendering have been slightly improved by adding the
--exact command line argument to the dvisvgm command. Previously,
characters near the image bounding box could be clipped.

The dvisvgm --scale was increased to 1.4 so that the font size better
match normal font size for text.

The Latex filter documentation have been updated to include information
about the SVG image format.
Updated references to the Latex filter

A couple of references to the Latex filter in the documentation was
updated to be image format neutral since the Latex filter now support
both PNG and SVG image generation.
Merge branch 'filter-latex-svg' of https://github.com/sm0svx/asciidoc into sm0svx-filter-latex-svg