conjurefile - support accessing remote addons

[adam-stokes]

This is to give the ability for someone to create a single addon in a git repo somewhere and define that in a conjurefile to be 'merged' in to the selected spell:

Currently, we define addons to execute that are already include in a spell:

    # (Optional) Set any addons you wish to deploy.
    #
    # For the Kubernetes spell you may want Helm installed so you can deploy
    # charts to the cluster.
    # addons:
    #  helm:
    #    01_install-helm:
    #      helm_version: v2.8.1

What we want here is to define a remote addon in addition to configuring it:

    # (Optional) Set any addons you wish to deploy.
    #
    # For the Kubernetes spell you may want Helm installed so you can deploy
    # charts to the cluster.
    # addons:
    #  helm:
    #    git-source: https://github.com/battlemidget/helm-addon.git
    #    01_install-helm:
    #      helm_version: v2.8.1