[WIP] SVG badges!

How it Works:

The top of a baked SVG badge looks something like this:

```svg
<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg"
     xmlns:openbadges="http://openbadges.org">
  <openbadges:assertion verify="http://example.org"><![CDATA[{verify:{"url":"http://example.org"}}]]></openbadges:assertion>
```

Note the `openbadges` namespace definition (resolved at http://openbadges.org because of reasons)

The `<openbadges:assertion>` tag has an attribute `verify` which mirrors the URL that would be baked into a hosted-assertion type badge. Additionally, the body of the entire assertion can be placed within the tag as serialized JSON wrapped in <![CDATA[ ]]>.

The code for extraction lives at a logical fork within the `extract` method, and `debake` remains unmodified as a result.

As the code currently stands, if the JSON is present, its `.verify.url` takes precedence over the `verify` attribute (though they should be the same)

- [x] Extracting assertions/ verification URL from SVG files
- [x] Command line flag `--svg`
- [ ] Baking assertions into SVG
- [ ] Testing new badge type