Add support for SVG->PNG conversion

[bwidawsk]

SVG is fraught with differences across web browsers. As such, thumbnail generation would benefit from converting to a more accepted format such as png. It is nice however to be able to link to the source SVG file in case people want to open in a different program and benefit from infinite scaling. Inkscape is the main driver for conversion as it seems to be the best currently.

This patch could use some work:

1. Currently won't work on Windows.
2. Inkscape 1.0+ is supported (older versions had different commandline invocation)
3. Could benefit quality knobs for the resulting png
4. High level config option to pick resulting output format (png, jpg, whatever)

I have no intention to fix any of those because they don't matter to me. I thought perhaps someone else could benefit from this and so I am submitting the PR as is.

My use of this is with shortcodes plugin. For example to have the "medium" thumbnail be displayed, but have it link to the svg can be done with the following
svg-image = '<div class="align{{ align }}">{% if link %}<a href="{{ link }}"{% if not link.startswith("http") %} data-lightbox="gallery"{% endif %}>{% endif %}{% set name,ext = image.split('.') %} <img src="{{ name+'-medium.png'}}">{% if link %}</a>{% endif %}{% if caption %}<span class="caption">{{ caption }}</span>{% endif %}</div>'

[skorokithakis]

This looks good, thanks! I'll test it shortly and merge if everything works well.