Use svgcheck as a Python library

[jayqi]

I would like to check an SVG file as a part of a unit test in a Python project, and being able to run an svgcheck(path) or svgcheck(text) function would be much nicer than having to use a CLI program with subprocess. Looking at svgcheck.run.main, it looks like there isn't currently a straightforward functional entrypoint, as all of the parameters (including source) can only be specified through command line arguments.