Allow per-TestConfiguration ICoverageConfiguration

[sewe]

At the moment, coverage configuration (from ICoverageConfiguration) is defined globally:

export interface IConfigurationWithGlobalOptions {
    /**
     * Test configurations to run.
     */
    tests: TestConfiguration[];
    /**
     * Configuration used for handling coverage.
     */
    coverage?: ICoverageConfiguration;
}

This is unfortunately a bit limiting, as one cannot collect coverage separately, e.g., for integration vs. unit tests defined in two separate TestConfigurations. (Of course, this would also require that #38 be fixed; it both configurations write to the same output directory, this won't work.)