Pull extension

Significant number of infrastructure components supposed to be reusable. Reusable component could be maintained in a central repository and embedded into stack's source code repository with Git substree while still maintaining link to upstream.

components:
- name: backend-ecr
  source:
    dir: components/ecr
    git:
      remote: 'https://github.com/agilestacks/components.git'
      ref: master
      subDir: ecr  # `git subtree split` will be used to extract relevant commits
- name: tls-host-controller
  source:
    dir: components/tls-host-controller
    git:
    remote: 'https://github.com/agilestacks/tls-host-controller.git'
    ref: master

Use hub pull to add and update subtrees:

hub pull
hub pull -c tls-host-controller
hub pull --show
hub pull --help