Add description as comment in crontab

[dyson]

Adds a desc to a job that will be output as a comment above the cron
entry in the crontab. It supports single line and multiline
descriptions.

[benlangfeld]

Looks great. Would you please add a changelog entry for this?

[serggl]

seems like this has been stale to awhile. Any chance to give it a go?

[taketo1113]

Thank you for your contribution!

The option name has been changed from desc to description as shown below:

-every 1.hours, desc: "My job description" do
+every 1.hours, description: "My job description" do
   command "/usr/bin/my_great_command"
 end