Terms Facet: Allow to provide `_index` as the field, resulting in facets on indices

[kimchy]

When searching across several indices, sometimes it make sense to return a facet that includes a count of hits per index. This is similar in notion to terms facet (consider the index name as if it was a field stored in the index).

A simple terms facet definition, with _index as the field name will return a facet containing the mentioned info.

[kimchy]

Terms Facet: Allow to provide _index as the field, resulting in facets on indices, closed by 4b87f63.