goreleaser fixes

Deps update & some cosmetic changes

Bugfix mapping coilgroups

Fixed, configurable polling speed for coil groups

Version number available as flag

Coverage measure + improvements

Systemic refactor polling groups of coils

Fix goreleaser deploy

Adds subscribe to topic for all writable coils

Regular modbus coil mappings mention coils to be either read-only or
read-write. For our purposes here, a mode of "write-only" is added to
assure that we only use the coil for writing to from an incoming MQTT
event. Otherwise, the following would happen:
- MQTT event triggers coil update
- coil update; gets polled
- on poll, the coil pushes out another MQTT event, effectively undoing
the previous change (or even worse, leading to some infinite loop
behavior, depending on the topic and values used).

Put in infinite loop to keep connection open;
[SO](https://stackoverflow.com/questions/48872360/golang-mqtt-publish-and-subscribe)
has a solution based on an OS signal.

Goreleaser docker push on tag

travis ci / goreleaser setup.