Allow filtering of sensu events and add test for filtering these events

Sensu does now have the ability to filter events based on query params. So we need a way to filter what sensu events we want in blinken. This PR attempts to do it by having a new config entry simillar to how the query params are specified where we can specify how to filter the events returned. This is currently only implemented for sensu as for icinga the query params could be used to achieve the same thing.