Allow for parallel "for" loops

[q0rban]

Currently dependencies run in parallel. With the introduction of looping over variables, it would also be nice to optionally have those jobs run in parallel.

If you agree, perhaps this syntax would be most intuitive (parallel: true inside the for object):

version: 3

tasks:
  default:
    vars:
      ITEMS:
        sh: echo {1..50}
    cmds:
      - for: { var: ITEMS, parallel: true }
        task: process-item
        vars:
          ITEM: '{{.ITEM}}'

  process-item:
    cmds:
      - sleep 5 
      - echo {{.ITEM}}

[andreynering]

I'll mark this as a duplicate of #1299.

Since dependencies run in parallel, both would have the same effect. Also, it's better to keep the discussion on a single place.