How to pass parameters to filters, readers and writers

[massifrg]

Sometimes I'd like to change a bit the behavior of a filter/writer of mine just changing some of the command line options of pandoc. In other words, I'd like to pass arguments to filters via the pandoc CLI.

These are the ways I know to pass parameters to filters/readers/writers:

• environment variables, that can be accessed through pandoc.system.environment() in lua;
  the only problem is that they must be set before you call pandoc;

• meta values with -M KEY[=VAL] or --metadata=KEY[:VAL] syntax on the command line;
  you may abuse that option to pass parameters to filters, but you must remove those meta values
  from the output doc's meta if that is their only goal;

• variables with -V KEY[=VAL] or --variable=KEY[:VAL], but they are meant for templates;

• (custom extensions names as a way to pass boolean values?)

Environment variables are the best fit, but I'd like an option like -V/--variable for lua filters/readers/writers.