Support sibling columns, with _cat/indices?pri as first go.

% curl localhost:9200/_cat/indices\?h=health,index,rep,docs.count,ss,cs,fm\&v\&pri
health index   rep docs.count      ss  pri.ss cs pri.cs     fm pri.fm
green  wiki      1       6401 302.9mb 151.4mb 0b     0b 25.1mb     0b
green  twitter   1      11434    64mb    32mb 0b     0b     0b     0b

Closes #4689.