Add SVG generation for LaTeXML

Currently SVGs are generated only in
MathML mode but not in LaTeXML mode.
Since LaTeXML can benefit from the
SVGs as well this change introduces
the functionality to generate SVG
images from LaTeXML MathML input.

Generating SVG images for LaTeXML is an essential
part of Math 2.0. It enables to include LaTeXMLs Content
MathML and provides fallback
images for visitors with MathML disabled browsers.

Change-Id: If13a8b0825bf12dbfe4920ddb7ad552df9bc095f