User can define api spec directly as object

[rwalle61]

Part of #31: allows user to load api spec like this:

before(async() => {
    const apiSpec = (await axios.get('url/to/openapi/spec')).data;
    chai.use(chaiResponseValidator(apiSpec)); // no need to write out to a file
}

Also:

• minor clarification of test blocks
• minor corrections to the README