More rendering params

[openvmp]

Add rendering params (both the package configuration file and cli command options) for strokeWidth, marginLeft and marginTop.
They need to be stored in the context class next to "output_dir".
Perhaps rename "output_dir" to "render_output_dir" and store new options in "render_...".
While output_dir is used in all rendering functions (see shape.py),
these three options will need to be used for generating the SVG file and, thus,
influence other formats produced from SVG (at least PNG for now, more to follow).