Added support for tags in the JSONFormatter

This small change updates the JSONFormatter to include tags on scenarios and their parent features. 

Tags are very useful when reporting on the status of a feature suite. It makes sense to expose these tags in the JSON output for other applications which might be parsing it.

Tags are included in features as follows:

    "features": [
        {
          "title": "A feature",
          "desc": "A feature description",
          "tags": [
            "json"
          ]

Tags are included in scenarios as follows:

    "scenarios":  
        {
          "title": "A scenario",
          "isOutline": false,
          "tags": [
            "json"
          ]