Multiple commands to get data

[tpluscode]

Similarly to how MULTIPLE_INPUT_JSON allows multiple command to pipe data to index, it would be useful to define multiple command to get data from various sources.

Re #117, YAML would make it much simpler with native support for arrays

data:
  get-data-cmd:
    - curl ... -o fetched-file.nq # from web
    - mc cp bucket/foobar.gz ./foobar.gz # from S3
    - sparql ... > results.ttl # result of a query

[hannahbast]

@tpluscode I agree that it would be very useful to have multiple get-date commands. It's a very frequent use case and the current long command lines are hard to read and maintain.