This commit allows to extract the metric name from the topic path. Now
it can be configured if all metrics are in a object in a certain topic
or if every topic contains exactly one metric. However, currently these
modes can not be mixed.
This should solve !26
TODO:
* Update documentation
* Add unit tests
The device firmware tasmota places the device ID in the middle of the
topic path. To support those devices to this commit introduce the
device_id_regex configuration. The old behavior of the last element of
the topic path is unchanged.