Feature: Make emphasis in definitions a different type

  - so that a separate color may be assigned to defined things
Feature: Allow to set prefix for images src attribute

  -- when image_path is set (either on the command line or from a file)
     all src attributes of images will be prefixed with image_path
Feature: Add 'Notation' to allowed block definitions.
Feature: Allow almost arbitrary definition blocks (i.e. not hardcoded)

  -- all must start with a capital letter and the rest consist of lower case
     letters
  -- also, they must not be proof - blocks
Feature: Implement basic support for macros.