5.0: Add constants for known date formatters

[niemyjski]

I think it would be helpful to provide some known constant values for things like date formatters: https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-date-format.html#built-in-date-formats

What are your thoughts on this? Also, should a default one be sent in elastic 5 (dateOptionalTime was sent in 1.x).

[russcam]

Known date formats seems like a good idea 👍

[gmarz]

+1 on adding a DateFormat enum. This means adding an overload everywhere we take a string though.

[Mpdreamz]

We now have constants on DateFormat.* thanks for the suggestion @niemyjski 👍