Gladtex

- [source only] move gladtex.py to gleetex/main.py

- fix AtributeError when specifying `-E`

- 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

- When formula replacement with `-R` is requested, it could happen that
additional spaces were inserted, even if not necessary. "für" would for
instance become "f\"{u} r". Fixed.

- It seems as if 16px / 12pt were the default font size these days for
browsers. Therefore, the default resolution has been set to 115 DPI.
Furthermore, the DPI switch now accepts pt values for fontsizes and will
calculate the corresponding DPI itself.
- When the environment variable `DEBUG=1` is set, the full backtrace will be
printed.
- Extend unicode table creation script to allow blacklisting of certain
commands.

- replace characters with diacritics in the LaTeX source, but keep the
unmodified character in image alt attribute (for better readibility)