add "conditional_entries" syntax

[myrdd]

suggested .json format extension: (example)

{
  "metadata":{
    "version":1,
    "serial":2017091501
  },
  "conditional_entries":[
    {
      "name": "uBlock origin",
      "conditions": [
        {
          "enabled_extensions": [
            {
              "id": "[email protected]",
              "type": "webextension"
            }
          ]
        }
      ],
      "entries":{
        "allow":[
          {
            "o":{"s":"moz-extension","h":"{extension_uuid}"},
            "d":{"s":"https","h":"*","port":"*"}
          }
        ]
      }
    }
  ]
}

or

{
  "metadata":{
    "version":1,
    "serial":2017091501
  },
  "conditional_entries":[
    {
      "name": "uBlock origin",
      "condition": {
        "enabled_extension": {
          "id": "[email protected]",
          "type": "webextension"
        }
      },
      "entries":{
        "allow":[
          {
            "o":{"s":"moz-extension","h":"{extension_uuid}"},
            "d":{"s":"https","h":"*","port":"*"}
          }
        ]
      }
    }
  ]
}

RP then needs to replace "{extension_uuid}" by the current UUID of the extension.

Btw, "conditional_entries" could be nested to allow for grouping.

I'm not sure if it's possible for WEs to retrieve the uuid. The uuid is stored in Extension.jsm [UUIDMap, Extension.uuid].

[myrdd]

implementation issue: RequestPolicyContinued/requestpolicy#846
WE API issue: RequestPolicyContinued/requestpolicy#845