Merge branch 'zh/trailer-cmd'

The way the command line specified by the trailer.<token>.command configuration variable receives the end-user supplied value was both error prone and misleading. An alternative to achieve the same goal in a safer and more intuitive way has been added, as the trailer.<token>.cmd configuration variable, to replace it. * zh/trailer-cmd: trailer: add new .cmd config option docs: correct descript of trailer.<token>.command