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.

Support for ~ in cachedir

• added support for ~ shortcut for cachedir
• fixed square brackets in documentation
• fixed handling of special characters in alt and title options (fixes #70)

Fixed rendering in indented blocks

Fixed rendering when using diagrams in indented blocks, such as lists.

Fixed pip installable packages

In release 3.1.1 there was an extra file in pip packages that that was preventing the plugin from working properly.
This new new release fixes the packages.

Fixed compatibility with Markdown 2

Fixed a compatibility issue that prevented the usage of the plugin with Markdown 2.x.