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).

Added support for image maps

When the output format is png and the diagrams contains an hyperlink, a map tag and related area tags are added to the output so the links are active.

Forced utf-8 encoding for source files

Now the character encoding for external files (referenced by the source parameter) is utf-8, even in Windows.
Use the encoding configuration option to force a different character encoding.

Fixed multiple mixed code blocks

Fixed parsing of mixed fenced_code and plantuml_markdown blocks in
the same document (see #45).

Minor changes

• fixed UML source diagram inside fenced code (refs #45)
• external source diagram through source option is now prepended to the inline code (both are kept) (refs #47)

Added support including diagrams from files

Now is possible to include diagram code from external files.
Fixed also a minor bug with the object HTML tag.

Removed annoying debug messages

Removed a forgotten instruction that was setting the log level to debug, causing some debug messages to be written in the output.

Tested with Python 3.8

Fixed the failing tests.

Added plugin priority configuration

Added the priority plugin confiiguration which can be used to fine tuning the plugin execution order.