Fix multiple command vulnerabilities

* The target host passed to :class:`foomodules.Commands.Host` and
  :class:`foomodules.Commands.Ping` could be crafted to possibly produce
  malicious behaviour, as it is passed verbatim to the respective shell
  command. Using e.g. `-mtrace` as hostname for the host command would
  produce considerable memory and cpu consumption.

* :class:`foomodules.Commands.Dig` allowed injection of a single flag
  (by setting it as host name) if a record type was set. In that case,
  `dig` would intepret the record type as host name to look up and
  the host name (starting with a `+`) would be taken as a flag.