Our test coverage is actually pretty decent, so now would be a good time to set up Codecov or coveralls for the project.

Some things we need to take a look at:

• We should mark hard-to-test code blocks with /* istanbul ignore next */
• echo tests appear to have horrible coverage because of how the tests are run. We'll need to resolve Echo tests unnecessarily run tests in own process #622 to make test coverage more accurate
• Some lines are untested but should be tested. These cases are pretty trivial, I can take these on.
• Some lines of code are actually dead code that are never reached. We might consider removing these at this point.
• Actually add Codecov to the project 😄

Blocked on #622