Add input/ouput filter nodes

[renne]

Currently each MQTT-input/output node has to be configured for the network setup. With a lot of MQTT nodes you have to type a lot of redundant information.

I suggest to add a MQTT-input and a MQTT-output to mosca and a node for MQTT message filtering. That way e.g. multiple inject nodes can be connected to the input of mosca and multiple filter nodes to the output of mosca.