Add option to filter out fields by removing unwanted ones

[sgratzl]

so far it is possible to limit the returned fields in a positive way by using the fields= parameter, e.g., fields=geo_value,value will only return these two columns. An alternative approach would be support negative fields, e.g. fields_exclude=stderr won't transfer stderr but everything else. Alternative syntax: prepending a minus, e.g., fields=-stderr