Add support for 16-bit/half floats in ES 5.x

[costin]

ES 5.x alpha4 introduced 16-bit float (elastic/elasticsearch#18887) which ES-Hadoop should start supporting.

[acchen97]

Additionally, in the vein of new data types, ES 5.0 is also splitting strings into text (analyzed field, no doc values) or keywords (non-analyzed, doc values enabled by default). elastic/elasticsearch#12394

@jbaiera what do you think about adding this? Happy to create a new issue as this type of stuff is probably worthwhile getting into the major.

[jbaiera]

@acchen97 It seems that support for the text and keyword types was added a while ago. Winner by default!

[acchen97]

Thanks for checking. I remember mentioning it a while back, but couldn't find the issue/PR for it right now.