Make title image configurable via Markdown option
The image is determined by the "title-image" option in the front-matter of the markdown file. It can either be the keyword "default" or the name of an image file. The image file is expected to be found in the image directory (c.f. _config.yml). Should the option be unset or the image not be found the blue background color as set in title-image.css will be used instead. To achieve this, the file "_config.yml" has introduced which holds basic configuration which applies to the whole site as to avoid magic strings.