Add message/warning to run sync when hosts on provider are changed after the last sync

[valentinocossar]

Add message/warning to run sync when hosts on provider are changed after the last sync. With this implementation, we aim to notify users when hosts are changed on the provider repository after they do the last sync, in this way a user knows when he has to run another sync to pull the latest hosts.

[CasvalDOT]

Okay, we can also add an extra option in the config file to allow user to set this behavior automatically.
What do you think @valentinocossar?
An example must be:

auto_sync_on_provider_change: true | false

Default can be false

[valentinocossar]

Yes, we can also add this feature. I would simplify the name with auto_sync: true | false. Default to false.