Provide an option to control the verbosity of dexec.Cmd

[thallgren]

The logging of dexec.Cmd() can be turned on or off using the DisableLogging flag, but there is no way to control how much output the command generates and it's generally very verbose. In many situations, all the reader of a log wants to know is if the command executed and if something went wrong.

I would like a way to control this so that only the command string is logged and, unless there's an error (output to stderr or non-zero exit code) nothing else.

[LukeShu]

See the discussion at #22 (comment)