'count' output 

Fixes #109.

This PR depends on #135, i.e. pull request 135 (csv output) needs to be applies first.

Example:


    [out:csv("@count";false)];
    area[name="Saarland"][boundary]->.a;
     (node(area.a)[amenity=pharmacy];
       way(area.a)[amenity=pharmacy];
       relation(area.a)[amenity=pharmacy];);
      out count;


Overpass API result:

`239`

In JSON/XML mode, fields "total", "nodes", "ways", "relations" and "areas" will be printed according to the occurrence in the input set.

XML Example:



    <?xml version="1.0" encoding="UTF-8"?>
    <osm version="0.6" generator="Overpass API">
    <note>The data included in this document is from www.openstreetmap.org. The data is made available under ODbL.</note>
    <meta osm_base=""/>

      <count total="3171" nodes="0" ways="0" relations="3171"/>

    </osm>

JSON Example:

    {
      "version": 0.6,
      "generator": "Overpass API",
      "osm3s": {
        "timestamp_osm_base": "",
        "copyright": "The data included in this document is from www.openstreetmap.org. The data is made available under ODbL."
      },
      "elements": [
    
    {
      "count": {
        "total": 3171,
        "nodes": 0,
        "ways": 0,
        "relations": 3171,
        "areas": 0
      }
    }
    
    
      ]
    }


In CSV mode these values are exposed through the following artificial keys:

`
@count, @count:nodes, @count:ways, @count:relations and @count:areas
`