Call DeleteWebhook before starting the polling loop

[PaulSonOfLars]

What

Call DeleteWebhook before starting the long polling loop, simplifying the long polling code.
Also, add a new DisableWebhookDeletion option, which allows you to manage webhook deletion yourself.

As a drive-by, improve docs and comments.

Impact

• Are your changes backwards compatible? No - changed the GetUpdateOpts in the StartPolling method to use a pointer instead of a value.
• Have you included documentation, or updated existing documentation? Yes
• Do errors and log messages provide enough context? Yes