How to specify input arguments for arbitrary shell scripts

[GeigerJ2]

cli_arguments:
    positional: [data1, data2]
    keyword:
        --input: obs_data
    flags:
        - --overwrite
    source_file: <path-to-file-that-should-be-sourced-before>

Keyword arguments are basically already being handled by input_arg_options, and flags by command_option, though, I find this naming very non-intutive, and would vouch for the one given above.

Note to self, the subclassing to ShellTask and IconTask is also being worked on in #59 and #62.