add a generic data bucket, so tests may store additional metadata

[catdad]

... this is useful when writing other tooling to look at results.

module.exports = {
    test: function (done) {
        ...
        this.data.someProp = 'value';
        ...
    }
};

• [x] add code and tests
• [x] add README topic
• [x] publish

[catdad]

Note #178. The usage of the afterAll function will not be fixed at this time.

[catdad]

Published in v0.8.0.