Extend meta data support

[bogdankostic]

Right now, we only allow strings to be used as meta data for filtering. In order to get better search results, we want to add the following:

• filtering for data types other than string, e.g. dates, ints, floats, bools
• filtering not only for exact match, but also comparisons (<, >, <=, ...), ranges of values (e.g. dates) etc.
• combining multiple filters with logical AND / OR operations

Related issues: #625, #1226, #1939

[SjSnowball]

@tholor @bogdankostic Extending metadata mapping and filtering support will bring great flexibility for keyword search.
Currently the list of fields passed in search_fields, content_fields are indexed as the "text" field. The rest of the fields are forcefully set to "Keyword".
Similarly, a new parameter will be introduced which will take the data_type and list of fields belonging to that ??

[
  { "date": [field1,field2] },
  { "keyword": [field3,field4] },
  ...
}

I would love to work on this issue.Please let me know