Add support for field-specific facet params.

[marc-portier]

We already have query.facet(options) to build up the 'general' solr-facet request params. However, many of those can also be overriden on a per field basis using the pattern:

• global param: facet.{paramname} =
• field-specific variant: f.{fieldname}.facet.{paramname} = ... See https://wiki.apache.org/solr/SimpleFacetParameters#Parameters

I see a number of options to introduce this. either embedded in the current options like is suggested in the facet-query test for the date and range fields:

options = { 
    "limit" : 5
    "fields" : [{
        "field": "myfield",
         "limit" : 10
        },
       ...
      ]
    };
q.facet(options)

or through an overloaded variant that takes both field-name and it's specific set of options

q.facet({"limit": 5})
  .facet("myfield", {"limit": 10})
  ...

Note: while checking up I notice that the facet.date and facet.range support is not yet there, so tackling that together with what is mentioned here is due.

[nicolasembleton]

@marc-portier That sounds like a good feature to add indeed... Would you mind adding those 3 stories (one per point) to the trello board?: https://trello.com/b/xCBvFQSe/solr-client-board We'll prioritize them.

Also if you have some real life example of how you guys are doing it (are you?) and how you'd implement them to feed the thoughts. Would be nice.