Conduit: dynamic init subcommand

[winder]

Extend the conduit init command to accept a list of plugins. Generate a skeleton configuration which includes those plugins.

~$ ./conduit init --plugins algod,block_evaluator,postgresql
...
~$ cat data/conduit.yml
Importer:
    Name: algod
    Config:
    - netaddr: ""
      token: ""
Processors:
  - Name: block_evaluator
    Config:
      - indexer-data-dir: ""
        algod-data-dir: ""
        algod-addr: ""
        algod-token: ""
        catchpoint: ""
Exporter:
  - Name: postgresql
    Config:
      - connection-string: ""
        max-conn: ""
        test: ""