Support for conditional requirement of options

[Hobart2967]

Is your feature request related to a problem? Please describe.
Depending on the options and args given I'd like to require additional options. For example, I have a build task that requires an option "name" in some cases, but not in all.

The structure for "when" in tasks is great already and looks suitable to the requiring of options, too.

Describe the solution you'd like

# ...
  build:
    usage: Builds a part of this product 
    args:
      artifact:
        usage: "Type of artifact to build"
        values:
          - app
          - lib
    options:
      name:
        usage: Name of the target to build
        required:
          when:
            - { artifact: 'lib' }
# ....