Added support for server-side C4 includes

Added the server_include_whilelist configuration which is a list of regular expressions used to define which files can be safely included
by the server.
Useful when using a Kroki server for remote diagram rendering.

Added retry on server errors or rate limits

Connection errors can occur when using remote servers to render diagrams, especially in the case of many diagrams.
These are usually connection errors due to limitations on the number of calls.
This release solves these kinds of problems.

Removed forgotten test code

Removed test code that caused problems with Windows.

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.

Fixed rendering of yaml structures with remote server

When rendering plantuml yaml diagrams on a remote server, the spaces were removed from the lines and the diagram was not drawn as expected.

Removed an unused `plantuml` import

Removed a forgotten import after removing the plantuml package.

Fixed external includes

Rendering via servers using external includes (as in AWS Hello World Icons did not work due to the url defined by a !define directive.

Also, removed dependecy from the plantuml package.

Fixed image map detection on Windows

On Windows the image map detection was not working properly cause the different end-of-line terminator.

Ffixed error with external plantuml server

When using an external PlantUML server to render diagrams, if a diagram has syntax errors and the remote server returns an error code (HTTP >= 400), and exception was thrown immediately stopping markdown parsing.
Now the error is intercepted and logged, and markdown can continue its work.

Removed unnecessary uuid lib requirement

This is a minor fix, remove the unnecessary lib uuid requirement .
Thanks to @borystua pointing it out (see #60).