Add SETTINGS parameter for index keys

I think it would be very useful to add `SETTINGS` parameter for index creation. With this option we could create more useful indexes like sorted index (https://www.elastic.co/blog/index-sorting-elasticsearch-6-0).

I imagine the creation process of sorted index could be like this (just a suggestion, I am not sure of it):
```
mgmt.buildIndex('borderIndex', Vertex.class).addKey(name, Settings.SORTED.asParameter())
.buildMixedIndex("search")
```

Related discussion:
https://groups.google.com/forum/#!searchin/janusgraph-users/elasticsearch$206%7Csort:date/janusgraph-users/dtBxqdR2qr0/ieXJdWaVAAAJ

Start with the buildIndex and buildMixedIndex API shown in the issue, then read the related JanusGraph user discussion about Elasticsearch index sorting. Determine how index-key parameters are currently represented and what SETTINGS support should cover; done means the requested setting can be supplied during index creation and its behavior is verified.

Written by the indexing model from the issue text.