Support for more variable types in yaml default files

With default files (via `--defaults` switch) only a limited set of environment variables is available for use in path related fields. A really common use case where one would convert `markdown` file to `docx` file with only extension being different in the filename (apart from path) is therefore not supported.

Supporting more variable types, similarly to how this is implemented for `Lua` filters via `CommonState`, would improve user experience since even fewer switches and logic for achieving desired behavior would be needed. By supporting access to the name of the first input file without extension, passed via command line, the following would be possible:

`pandoc --defaults=my_defaults.yaml my_document.md`

After executing only the command above file `my_document.docx` could be created in `TEMP` subfolder of the folder where `my_document.md` is located.

Start by tracing handling for the `--defaults` switch and compare it with how variable access is implemented for Lua filters through `CommonState`. Check how the first command-line input file is represented and what additional variable types would be needed. Done means the shown command can derive `my_document.docx` in the `TEMP` subfolder without extra switches or logic.

Written by the indexing model from the issue text.