Idea: Adding the possibility to create a Dataframe from a KQL query

[bartbroere]

This PR is a first draft of how we could create a DataFrame by filtering an index with Kibana Query Language.

I found the kql module elsewhere in the Elastic Github organisation. For now I submodule+symlinked it in here, but there probably is a better way to do that.

You would use this by doing something like this:

import eland

eland.Dataframe('http://localhost:9200', 'example-index').kql_query('field1 > 100 and (field2:"value1" or field2:"value2")')

[bartbroere]

I'll be mostly offline during April, so this is just a heads up that this PR will not see much progress for a few weeks. I didn't forget about it though, and plan to continue working on it in May.