Ability to map @Id property to a field in the index (other than _id)

[rbygrave]

So currently the @id property is automatically mapped to _id in ElasticSearch. The issue there is that _id is always treated as a string type and can't be used in order by clauses etc.

What we want is a nice simple way to define that the @id property should additionally be made an explicit field in the index (and if it is numeric it should be indexed as numeric etc).