Extend CSV inventory to support per-instance configurations

[wagoodman]

As hinted to in #9 (comment) , it would be great to be able to map arbitrary CSV columns to instance parameters that override the global or inventory ssh configuration. For example, a CSV inventory with extra columns:

name, address, user, ssh-key
server-1, 123.87.123.223, admin, id_rsa.ss1
awesome-server-2, 22.54.21.123, frank, id_dsa.key42

Bridgy could take advantage of the extra information like so:

inventory:
  source:
    - type: csv
      name: on-site servers
      file: somefile.csv
      fields: name, address, user, ssh_key
      ssh:
        user: fields['user']
        options: -i fields['ssh_key']

The yaml format needs more thought, but that's the general idea (I'm open to suggestions).