Enhance metafacture CLI/`Flux.main()` in order to provide options for e.g. scripts, script files

[TobiasNx]

Based on a discussion with @blackwinter a while ago:

He suggested that Flux.main() could learn options, e.g.:

-e script for flux workflow as string (currently not possible)
-f script-file for flux workflow as file (currently first argument)
-v var=value (repeatable) for assigning variables (currently all other arguments)
-l/--list-commands for listing available commands (currently when no arguments given)

At least the first option would enable the CLI user to write one line in a CLI for a transformation

[blackwinter]

See also #271.

[blackwinter]

Functional review: @TobiasNx
Code review: @dr0i