Support special search parameter _filter

[thomas-rudlof-bl]

Currently, the special search parameter _filter is not recognized. Trying to use it results in

{
    "resourceType": "OperationOutcome",
    "id": "ac-12-0-4-fdb322b6-633d-475c-a0c0-a85fc8bec50a",
    "issue": [
        {
            "severity": "fatal",
            "code": "invalid",
            "details": {
                "text": "Search parameter '_filter' for resource type 'Patient' was not found."
            },
            "expression": [
                "<empty>"
            ]
        }
    ]
}

Would be nice if it was supported as described in http:https://hl7.org/fhir/search_filter.html