Support of variables and foreach

[Hobart2967]

Is your feature request related to a problem? Please describe.
Id like to see an option to set variables like real option values as shared variable.

Example:

vars:
  enabled-environments: &enabled-environments
    - dev
    - prod
    - all

tasks:
  build:
    options:
      environment:
        values: *enabled-environments
    run:
      - when: 
            equal: { environment: 'all' }
         task: build-all
      - when:
          not-equal: { environment: 'all' }
        run:
           yarn build:${environment}
  build-all:
    run:
      - foreach:
           list: ${vars.enabled-environments}
           as: current-environment
           task: # or run:
              name: build
              options:
                 environment: ${current-environment}

Describe the solution you'd like
This makes it easier to build a cli with an include all or specific option.

[rliebz]

Thanks for opening an issue!

I see this is as related to https://github.com/rliebz/tusk/issues/26—a proper array type should eventually support both the "for each" construct, to let users call something once per item, and serialization, to let users input values as arrays into Tusk while passing them along to other programs.