SVG, Pandoc filtering And More

• new features and incompatible changes:
  • add -P command-line switch to be used as a Pandoc document filter,
    see https://pandoc.org/filters.html
  • add environment variable GLADTEX_ARGS to pass command-line
    switches when used as pandocfilter where passing additional
    arguments is impossible
  • redefine colour handling: use xcolor package, therefore handling
    text and background colour the same way for both PNG and SVG
  • add SVG support for scalable images
  • use SVG output by default
  • gleetex.htmlhandling.HtmlImageFormatter: rename link_path to
    link_prefix
• bug fixes:
  • correctly parse HTML5 file encoding declarations
  • add more exceptions to the unicode table for the unicode replacement
    mode (see -R)
  • treat -d as a relative path