Please support svgz

Hello,
I'm currently using pygal to generate *svg* files, unfortunately, it makes my webpages pretty heavy (several dozen of megabytes). It would be super-great to implement [svgz support]( https://en.wikipedia.org/wiki/Scalable_Vector_Graphics#Compression ): `gzip` the svg content, and use the right content-type.

Since svg is xml, the compression is supper-efficient.

As for the implementation, shall I simply add a parameter to the `__init__` method to enable such thing, or is it a preferred way?

The issue names no files or tests. Start by locating pygal's SVG generation and output entry points, then check how content types are selected. Done means SVG output can be gzip-compressed as SVGZ and is served with the appropriate content type.

Written by the indexing model from the issue text.