added option for custom elasticsearch queries and filters

[stedomedo]

Added an option for custom queries and filters for elasticsearch.

An example filter can look like this:

filter = {"must": [{"term": {"tag": "irgendwas"}}, 
                   {"range": {"date": {"gte": "2020-01-01T00:00:00+0200"}}}]}

An example custom text query can look like this:

query = {"should": [{"match": {"text1": {"query": question, "boost": 2}}},
                    {"match": {"text2": question}}]}

[tholor]

Great, thanks for adding this @stedomedo!

Allowing custom queries and direct filters make a lot of sense.
We might also get rid of the current pre-filtering approach soon, as it doesn't make sense with Elasticsearch. It's mainly there for compatibility with current SQL-based documentstores.

I added a comment regarding the "terms" vs. "term" issue (and updated the docs). Can you please check if this fixes your issue?