Add new configuration format for multi map

Add new configuration format for multi map.
New format is here,
```
<match tag>
  type map
  format multimap
  mmap1 [tag + ".a", time, {"value" => record["a"], "key" => "a"}]
  mmap2 [tag + ".b", time, {"value" => record["b"], "key" => "b"}]
  mmap3 [tag + ".c", time, {"value" => record["c"], "key" => "c"}]
  ...
  mmapN [tag + ".z", time, {"value" => record["z"], "key" => "z"}]
</match>
```

Add new parameter "format".
It is a mandatory specification of the "format" parameter to use the new format,
but the existing configuration can be used as before without "format" parameter.