Gzips svg assets and creates a new file with svgz extension

Since SVG images are essentially XML, they could benefit greatly from Gzip compression resulting is over 80% compression.

This pull request allows for the Gzip of SVG files and saves them with the .svgz extension, as per convention. This is different from the usual .gz extension used in .css and .js files.

The original file is also retained of course.