Support --citeproc in pandoc

[pbsds]

pandoc-citeproc has been deprecated and removed in favor of the built-in flag --citeproc.
This option is to my knowledge currently not possible to pass in to pandoc in markdown-preview-plus.

Suggested fix:

Either perform a version detection on pandoc and use --citeproc or -F pandoc-citeproc accordingly, or simply add a new option to use --citeproc instead.

[pbsds]

Current workaround:

# defaults.yaml, loaded with --defaults defaults.yaml
filters:
- type: citeproc

This has the side-effect of the preview not working if a defaults.yaml file is not present

[lierdakil]

This option is to my knowledge currently not possible to pass in to pandoc in markdown-preview-plus.

It is, manually via Commandline Arguments option. I'm not sure when I'll be able to get to this, but pull requests are always welcome.