Parameterize hard-coded class names

[jmcarp]

We've moved many hard-coded class names into configurable selectors, but we still have some hard-coded names. Even better would be removing these if we don't need them. A few examples:

• term--highlight
• accordion--collapsed
• accordion__button

cc @noahmanger

[noahmanger]

👍 I can look at this.

[shawnbot]

Here's a crazy idea: can we nix the class names altogether and use aria attributes instead? For instance:

• term: role="definition"
• highlighted term: aria-expanded="true"
• accordion button: role="button" and aria-controls="content"
• collapsed accordion: aria-expanded="false"

[noahmanger]

So I think I mostly addressed this here: #6

We don't need .accordion--collapsed or .accordion--button, but .term--highlight is useful because it's a presentation class that's applied to the term in the body when it's clicked in order to highlight it.

[noahmanger]

Only thing I don't know is if we want to accept a hash of class names that people could supply in order to style aspects of the glossary.