Change dst/src dict keys to -/+ keys

[glennmatthews]

See #34. I think it's an improvement but would appreciate input!

Diff dict before:

{'location': {'DC1': {'device': {'DC1-LEAF1': {'interface': {'Ethernet1/3': {'_dst': {'mode': 'NONE', 'switchport_mode': 'NONE'},
                                                                             '_src': {'mode': 'TRUNK', 'switchport_mode': 'TRUNK'}},
                                                             'Ethernet1/4': {'_dst': {'mode': 'NONE', 'switchport_mode': 'NONE'},
                                                                             '_src': {'mode': 'TRUNK', 'switchport_mode': 'TRUNK'}},
                                                             'Ethernet1/49': {'_dst': {'mode': 'NONE', 'switchport_mode': 'NONE'},
                                                                              '_src': {'mode': 'TRUNK', 'switchport_mode': 'TRUNK'}},

Diff element dict before:

{
  "dst": {
    "mode": "NONE",
    "switchport_mode": "NONE"
  },
  "src": {
    "mode": "TRUNK",
    "switchport_mode": "TRUNK"
  }
}

Diff dict after:

{'location': {'DC1': {'device': {'DC1-LEAF1': {'interface': {'Ethernet1/3': {'<': {'mode': 'NONE', 'switchport_mode': 'NONE'},
                                                                             '>': {'mode': 'TRUNK', 'switchport_mode': 'TRUNK'}},
                                                             'Ethernet1/4': {'<': {'mode': 'NONE', 'switchport_mode': 'NONE'},
                                                                             '>': {'mode': 'TRUNK', 'switchport_mode': 'TRUNK'}},
                                                             'Ethernet1/49': {'<': {'mode': 'NONE', 'switchport_mode': 'NONE'},
                                                                              '>': {'mode': 'TRUNK', 'switchport_mode': 'TRUNK'}}}}}}}}

Diff element dict after:

{
  "<": {
    "mode": "NONE",
    "switchport_mode": "NONE"
  },
  ">": {
    "mode": "TRUNK",
    "switchport_mode": "TRUNK"
  }
}

[dgarros]

LGTM

[glennmatthews]

One aesthetic downside is that while < sorts before >, - sorts after +:

>>> sorted(["<", ">", "-", "+"])
['+', '-', '<', '>']

This is mostly noticeable when using pprint as it sorts dict keys by default:

{'site': {'atl': {'device': {'atl-spine1': {'interface': {'eth0': {'-': {'description': 'Interface 0'}},
                                                          'eth1': {'-': {'description': 'Interface 1'}}}},
                             'atl-spine2': {'interface': {'eth0': {'-': {'description': 'Interface 0'}},
                                                          'eth1': {'-': {'description': 'Interface 1'}}}}}},
          'nyc': {'device': {'nyc-spine1': {'interface': {'eth0': {'+': {'description': 'Interface 0'},
                                                                   '-': {'description': 'Interface 0/0'}}}}}},
          'sfo': {'device': {'sfo-spine2': {'interface': {'eth2': {'+': {'description': 'Interface 2'}}}}}}}}

[dgarros]

LGTM