Merge commands from runfile-exec

[DannyBen]

This PR takes all the commands that are available in the runfile-exec gem and integrates them directly into Runfile.

By design, the commands added by this module are not automatically included in the runfile.

it adds commands such as:

run 'any external command'
run_bg 'command'
run_bg 'command', pid: 'easy-to-remember-name'
run_bg 'command', log: 'logfile.log'
stop_bg 'easy-to-remember-name'

To use it, just add

include Runfile::Exec

to your Runfile