Infrastructure: Add coverage report of roles, properties, and states …

…(pull #1124)

Resolves issue #1123 by adding a script that creates an `index.html` file in a `coverage` directory.

The index.html file includes 2 lists and 4 tables:
1. Roles with no guidance or examples (list)
1. Roles with at least one guidance or example
1. Roles with more than one guidance or example
1. Properties and states with no guidance or examples (list)
1. Properties and states with at least one guidance or example
1. Properties and states with more than one guidance or example

It also generates two CSV files -- one for roles and one for properties and states.  These files are linked from the HTML file and are also in the coverage directory.

The script can be run with the command:

`node scripts/coverage.js`

The script looks for code tags in headings to determine if a section contains guidance for a role, state, or property.
If those tags are not present, the following attributes can be added to the heading tag to indicate the presence of guidance for an aria role, property or state:

* `data-aria-roles` : Space separated role values on a heading in the practices document
* `data-aria-props`: Space separated property and state values on a heading in the practices document