Conduit: list subcommands

[winder]

A subcommand to list all of the plugins, grouped by type.

list - use the metadata object to get the description

~$ ./conduit list
Importers:
    algod            Importer for fet5ching blocks from an algod REST API.
    
Processors:
    block_processor ...
    filter          ...

Exporters:
    file_writer ..
    postgresql  ..

*list " - details. Not exactly sure how this works, maybe we should add a sample config in each plugin directory and include that as part of the metadata.

~$ ./conduit list algod
    Name: algod
    Config:
        - netaddr: "Algod netaddr string"
          token: "...."