Add Configuration.BaseUrl

[yashaka]

With a simple usage like in selenidejs version:

    async open(relativeOrAbsoluteUrl: string): Promise<Browser> {
        // ...

        const absoluteUrl = isAbsoluteUrl(relativeOrAbsoluteUrl) ?
            relativeOrAbsoluteUrl :
            this.configuration.baseUrl + relativeOrAbsoluteUrl;

        await this.driver.get(absoluteUrl);
        return this;
    }

Cover with at least one test using this option and then opening page with relative url.