Add format helpers for messages

[Edoardo Ottavianelli]

Please describe your feature request:

Add these format helpers:

• {{datetime}} -> 01-02-2006 15:04:05-0700
• {{date}} -> 01-02-2006
• {{time}} -> 15:04:05-0700
• {{count}} -> 1,2,3 for each message (when multiple messages must be sent, one counter for each provider and ID)

Describe the use case of this feature:

Use the format helpers above in messages.