Use String for delay field

[JoeJoeTV]

This PR changes the delay field to use a String value instead of a u64.

This has two reasons:

1. Requests using a u64 directly seem to be rejected with a 400 - Bad Request by both https://ntfy.sh and my own ntfy server
2. The delay option can also be specified as a string (e.g. 30min) for ntfy. This exposes this functionality through the crate.

[yukibtc]

Thanks again. Rebased, squashed and merged at e3d607f