Add _cat/allocation/{nodes}.

Visualizes how shards distribute across the cluster.

% curl localhost:9200/_cat/allocation
shards diskUsed diskAvail diskRatio ip        node
     2   85.6gb   379.4gb     22.6% 127.0.0.1 MacTaggert, Moira
     2   85.6gb   379.4gb     22.6% 127.0.0.1 Jarvis, Edwin

% curl localhost:9200/_cat/allocation/Jar\*
shards diskUsed diskAvail diskRatio ip        node
     2   85.6gb   379.4gb     22.6% 127.0.0.1 Jarvis, Edwin