Add support for URL prefixes for CSS and scripts.

JMarkdown2DeckJs uses relative URLs for the CSS and JavaScript files by
default. You can create HTML files with different URLs by providing a
configuration with a prefix for the URLs.

    import com.github.stefanbirkner.jmarkdown2deckjs.*;

    Configuration configuration = new Configuration("http://your.domain/");
    String markdown = readMarkdown(); //your code that reads the markdown
    String deckJsHtml = new JMarkdown2DeckJs(configuration).convert(markdown);