Add rename/reindex endpoint

[JBGruber]

There is currently no way to change the name of an index once created. It would be nice to use elastic's reindex API to create a new endpoint. The API can also be used to combine multiple indexes, which could be really useful:

POST _reindex
{
  "source": {
    "index": ["twitter", "blog"]
  },
  "dest": {
    "index": "all_together"
  }
}

[vanatteveldt]

also, this could give an option to change field mappings.