Suggestion: timer functionality

Hello.

When I want to schedule something transient I have two options:
1. sleep:
`sleep 1m`
`sleep 2hour`
2. systemd transient timers as
`systemd-run --on-active="12h 30m" --unit someunit.service`

It would be awesome to incorporate a same functionality into wait-on to have one tool to "wait". Like:

`wait-on 12:30`
`wait-on Tuesday`
`wait-on 10 minutes`

Parsing can be done with https://www.npmjs.com/package/chrono-node

I can make PR if you will merge