feature!/eleventy-img shortcode

[dnnsjrng]

• this implements the @11ty/eleventy-img plugin as a shortcode for twig
• due to necessary new config options I changed the userOptions properties
• extract shortcodes to separate files to keep the main plugin/shortcode file clean
• update the readme accordingly, highlight the features in a separate section at the top, add shortcode descriptions
• asset_path now returns just the userOptions.assets.base folder

[dnnsjrng]

To not increase the complexity and size of this PR, I would suggest to give those topics:

• width feature!/eleventy-img shortcode #25 (comment)
• more sophisticated folder structure for assets feature!/eleventy-img shortcode #25 (comment)
• additionalAttributes feature!/eleventy-img shortcode #25 (comment)
• shortcode properties as object feature!/eleventy-img shortcode #25 (comment)

... to their own feature requests and move them to separate PRs. Agreed?