CSS styling the HTML representation of InferenceData

[sethaxen]

We use <details> to enclose the plaintext representation of the groups in the custom InferenceData HTML show method. By default, the group name is styled with the right-facing arrow to the far left, which rotates to point down when the group is expanded. However, the CSS of the page in which the InferenceData's HTML is inserted might custom style the <details> to remove the arrow. Jupyter does this, for example.

We should embed whatever CSS is necessary in a <style> block after the HTML to ensure that at least in Jupyter the arrow is shown correctly, but it's better if we can style the <details> to show the error no matter what the global CSS is.

Related, each of our tags should probably have a custom class associated with it, which would allow others to style our HTML as they prefer.

[sethaxen]

Relates #10