Remove nbdime-conflicts metadata

[psychemedia]

Running the nbdime notebook differencer might leave associated metadata in a notebook cell of the form:

"metadata": {
    "nbdime-conflicts": {
     "local_diff": [
      {
       "key": "tags",
       "op": "add",
       "value": [
        "folium-map"
       ]
      }
     ],
     "remote_diff": [
      {
       "key": "tags",
       "op": "add",
       "value": [
        "nbval-ignore-output",
        "folium-map"
       ]
      }
     ]
    }

Create a simple CLI command to strip nbdime metadata from a set of notebooks. (This might also generalise to letting the user pass in a top-level metadata key and stripping that out.)