Support for extra columns in count query

This commit adds support for multiple filter types. It's still need needs, but I'm opening the PR to create a discussion about it.

The changes make this query: 

    GGRC.Models.Search.findOne({
      q : '', 
      counts_only:true, 
      extra_columns: "Workflow_All=Workflow,Workflow_Active=Workflow,Workflow_Draft=Workflow,Workflow_Inactive=Workflow",
      extra_params: 'Workflow:status=Active;Workflow_Active:status=Active;Workflow_Draft:status=Draft;Workflow_Inactive:status=Inactive'
    }).then(function(){console.log(arguments[0].counts)});

Return: 

    {
      Audit: 237
      ...
      Workflow: 18
      Workflow_Active: 18
      Workflow_All: 84
      Workflow_Draft: 40
      Workflow_Inactive: 26
    }

Which is what we need to make filters display counts.
By the way, @danring were you thinking of implementing extra_params in a similar way as this?