Execpath array accepts variable for dynamic patch configuration

[veenone]

The newly introduced execpath with array input is a great way to manage plantuml jar for document generation. However currently we need to hardcode the path of plantuml jar as below :

--- 
diagram: 
  engine: 
    plantuml: 
      execpath: ['java', '-jar', 'c:/tools/plantuml.jar']
  mime-types: 
    application/pdf: false 
    image/png: true 

...

My request is to enable dynamic path configuration, maybe using variable so the path can be configured somewhere else (i.e: in make file, environment variable..)

Maybe the configuration will look like this:

--- 
diagram: 
  engine: 
    plantuml: 
      execpath: ['java', '-jar', $(TOOL_DIR)'/plantuml.jar']
  mime-types: 
    application/pdf: false 
    image/png: true 

...

Or it can be passed in pandoc commandline:

Pandoc --metadata=diagram.engine.plantuml.execpath: '"java","-jar","$(TOOL_DIR)/plantuml.jar"'

[tarleb]

Related issue: jgm/pandoc#10252