Accept more options for target inputs and outputs

[fbecart]

Žinoma currently accepts an array of input paths to make a target incremental.

Pragmatically, these inputs could also be:

• environment variables
• outputs of commands
• response body of HTTP requests
• ...

While avoiding making the API too complex, we should make space for more options.

[fbecart]

Here is a proposed format:

target:
  build_docker:
    inputs:
      - { type: env_var, var_name: DOCKER_VERSION }
      - { type: files, path: . }
    outputs:
      - { type: cmd_stdout, cmd: 'docker image ls todos/backend:latest --format "{{.ID}}"' }

[fbecart]

Another possibility:

target:
  build_docker:
    inputs:
      - env_var: DOCKER_VERSION
      - fs_path: .
    outputs:
      - cmd_stdout: docker image ls todos/backend:latest --format "{{.ID}}"