Add table concatenation.
Add associative lookup of columns for arbitrary (and more intuitive) ordering.

  % curl 'localhost:9200/_cat/nodes?v&headers=jdk,ip,name'
  jdk      ip        name
  1.7.0_40 127.0.0.1 Mordo, Karl

Closes #4433.
Turn off some columns by default in cat/nodes.
Round load avg in _cat/nodes.
Add column separator for help output.
[DOCS] Add headers and help parameters.