Add possibility to generate payload dynamically

[shvydky]

In my case, it is required to send different payloads in every broadcast.

I propose to have an optional function pointer in Settings that will return payload on demand. In case when this pointer is nil the implementation should fallback to Payload field.

I will try to implement this in the fork and create PR.

@schollz, please let me know what do you think about my idea?

[schollz]

Sounds great! Feel free to add