Add a coverage option

[scboesch]

Provide an option to pass-in a coverage parameter that causes the node process to execute the coverage command rather than the test command.

{coverage=0.5}
The target use case for this function is to enable users to only be able to edit a test file for a project to increase the code coverage of the project. So rather than writing source code to pass existing tests, users will be writing test code to increase the test coverage of existing source code.

The built-in GUI should provide a way to execute the npm run coverage process in a similar way that the "Run Tests" button currently executes the npm test process.