Add support to configure facet, index and show fields to inject addition...

...al field-specific solr parameters

Fixes #759.

e.g.:

```ruby
config.add_index_field 'an_index_field', solr_params: { 'hl.alternativeField' => 'field_x'}
config.add_show_field 'a_show_field', solr_params: { 'hl.alternativeField' => 'field_y'}
config.add_field_configuration_to_solr_request!
config.add_facet_field 'some-field', solr_params: { 'facet.mincount' =>15 }
config.add_facet_fields_to_solr_request!
```

Will add:

   f.an_index_field.hl.alternativeField=field_x&
   f.a_show_field.hl.alternativeField=field_y&
   f.some-field.facet.mincount=15

To the query sent to solr.