Suggestion: --command / -c attribute

If I want to schedule something which will take sometime can't close session or write some boring boilerplate like:

`bash -c 'wait-on mysite.com && do something' & disown`

And because I relate on status from wait-on I can't use `bg` or `wait`. But if wait-on had --command attribute which will run in daemonized/background mode I don't even need them:

`wait-on condition -c 'do something'`

As a proposed solution for: https://github.com/jeffbski/wait-on/issues/16