Add ability to specify an array of providers

This adds a new `providers` array key to the ConfigBuilder model. Having an
array allows a single role to configure multiple providers:

    ---
    roles:
      1gb-memory:
        providers:
          - type: virtualbox
            customize:
              - [modifyvm, !ruby/sym id, '--memory', 1024]
          - type: vmware_fusion
            vmx:
              memsize: 1024

Closes GH-19.