Add {path} keyword to dynamic templates to be replaced with the full path of the field

For a project I've been working on I have json documents that contain multiple inner objects, which contain similar fields, as in the following example:

```
{
    "inner1" : {
        "title" : "title inner1" 
    },
    "inner2" : {
        "title" : "title inner2"
    }
}
```

I want to keep all the titles in separate fields `inner1.title` and `inner2.title`, but at the same time I want to have a catch all field that contains all the titles, possibly as a top level field called `title`. I achieved this with a multi_field using the `just_name` path like this:

```
"inner1" : {
    "type" : "object",
    "properties" : {
        "title" : {
            "type" : "multi_field",
            "path" : "just_name",
            "fields" : {
                "title" : {"type":"string"},
                "inner1.title" : {"type":"string"}
            }
        }
    }
}
```

It is tricky because it's exactly the opposite of what a normal multi_field would do, since the first title (without path)  becomes the top level catch all, while the other `inner1.title` is the usual `inner1.title` field. Since I have to repeat in the mapping the same multi_field for every level where I want to take the title from, I decided to use dynamic templates to keep the mapping to submit when creating the index more concise. Unfortunately the fact that the second field in the multi_field contains the path is a problem and not supported through a placeholder like {name} or {dynamic_type}.

The pull request adds the support for a new {path} placeholder that makes it possible to do the following:

```
"template":{
    "path_match":"*.title",
    "mapping":{
        "type": "multi_field",
        "path":"just_name",
        "fields":{
            "{name}": {"type": "{dynamic_type}"},
            "{path}": {"type": "{dynamic_type}"}
        }
    }
}
```

This way with a single dynamic template I can achieve what I want for multiple fields taken from multiple levels in the json tree.