Added Kroki server support

With this release a Kroki server can be used as replacement for a PlantUML server for remote image rendering.
In the case of a Kroki server there are a couple o different behaviors:

• image maps are note (yet) supported by the server, disable them with the image_maps configuration
• Kroki does not render an image with syntax errors; instead, a text with kroki's error message is inserted in place of the image.