Properly detect non-SVG image size

#20

* New method of checking if the image is supported or not,
* Simplify some code
* Fixed displaying of "width" and "height" for unsupported images
* Extract width and height attributes from SVG as well (untested)
* Reindent `package.json`

This comes at the cost of removing complete support for SVG support. That's fine, though. I can write a separate routine to handle SVG images, and at the same time improve width/height detection accuracy via `src/SvgSize`. This issue is already tracked in #15.