Add /_cat/fielddata to display fielddata on a per-node per-field basis

[dakrone]

This would be great for figuring out where all your memory is going with regard to field data:

$ curl localhost:9200/_cat/fielddata
node   body  timestamp   subject  myotherfield  total
node1  7gb       501mb     1.1gb          17mb   10gb
node2  1.3gb     100mb     981mb           2mb  3.1gb
node3  2.7gb      81mb     182mb         512kb  3.2gb

It would be a nice-to-have for 1.0 RC1, but not required.