Add an option for targets to specify a log file

[xmnlab]

It would be nice to have an option to specify a log for for a target. also, it would be nice to define the level for the log (stdout, stderr, or both). For example:

groups:
  mygroup:
    targets:
       mytarget:
         log:
           path: /tmp/log.txt
           level: err  # err, out, both

[xmnlab]

some explanation about output/error redirection with bash: https://linuxize.com/post/bash-redirect-stderr-stdout/