Add support for Count requests

[ido50]

This commit adds initial support for Count requests, which are simple search requests asking to get the number of matches for a query.

The functionality is provided by the Count() function, which accepts a query object (implementing the Mappable interface), and can be executed just like a search request.

res, err := esquery.
    Count(esquery.Match("user", "someone")).
    Run(es)