Suggestion: Field syntax

[anthonycook]

Hey, I'm new to this library so forgive me If I've missed something but the syntax for querying multiple fields is:

	query := elastic.NewQueryStringQuery("example AND search")
	query = query.Field("title")
	query = query.Field("description")
	query = query.Field("tags")

I imagine this syntax could get quite large/repetitive with a large number of query fields, so could would It make sense to have something like Fields that accepts a multiple strings:

	query := elastic.NewQueryStringQuery("example AND search")
	query = query.Fields("title", "description", "tags")

[olivere]

Hey. It makes sense to change the Field(field string) methods to a signature like Field(field ...string) which allows all of the above without a breaking change (in terms of recompilation at least).