Add ability to create hooks

[JrGoodle]

They can be stored in the clowder repo under hooks/. Think about places where it makes sense to run them.

Some possibilities:

• pre-herd
• post-herd
• pre-reset
• post-reset
• pre-checkout
• post-checkout
• pre-stash
• post-stash
• pre-start
• post-start
• pre-clean
• post-clean
• pre-prune
• post-prune

[JrGoodle]

could allow specifying in yaml file, something like:

hooks:
  herd:
    before: something
    after: something_else
  status:
    before: something
    after: something_else