Add code coverage action

[ghost]

Once #19 is merged, we'll need to add a workflow to upload results to https://coveralls.io.

There's an existing GH action to implement this (https://github.com/marketplace/actions/coveralls-github-action), but there are some additional configurations to consider.

• update .coveralls.yml with the new repo token
• ensure that the default github.token is sufficient for our use case
• update the badges in README.md to point to the appropriate endpoint

Also, this step should probably run only on PRs into master or immediately after a PR is merged into master.