TODO

- Support multiple beamer-headers (simply pass all
  header files to pandoc)
- cleanup generated code files

- Replace the options --beamer-header, --html-header,
  --lua-filter, --syntax-theme and the new option for
  turning off auto shrink with a config file. The config
  file is a json file that allows setting all options.
  The config file is searched like this:
  * Name given on cmdline
  * File text-slides.cfg next to input file
  * File text-slides.cfg in ~/.text-slides

  File names in the config file are resolved relatively to
  this config file.

  Configuration options are resolved like this:

  1. Config file whose name was given on the cmdline. If
     no such config file was given, file text-slides.cfg
     in the dir next to the input-file
  2. ~/.text-slides/text-slides.cfg
  3. Default filename in ~/.text-slides (if applicable)
  4. The global default