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