OnlyIfBanned flag for unbanChatMember telegram API method

[slawiko]

Addresses #12

Adds --telegram.preserve-unbanned flag for avoiding situation when user is removed from group after unban. Also updates docs accordingly.

Telegram API docs for reference.

[umputun]

Two things:

• the value of KeepBanned was inverted in the original PR, setting it to true if preserve-unbanned was not set (false). This is likely a bug; I have flipped it. In case it was intentional, I don't get the intention behind this, and it also broke compatibility.
• The more important part: With this flag set, the bot API doesn't seem to unban users. Have you tried to use it in real life? Because if this prevent unban like I have seen, this flage doesn't work as expected and should be removed

[slawiko]

about flag inverting - my bad, did few iterations of renaming and confused myself. Thanks for fixing

about the unban process not working - again my bad, it is not working indeed. I investigated why, and the reason is that user is not actually banned, but restricted. Previously, without only_if_banned flag, we did not see issues with unbanning, because user was removed from the group together with its restrictions.

Of course we can get rid of that flag and everything will work as expected before. But also we could really ban user instead of restricting it (I prepared PR). This way the preserve-unbanned flag will not be needed, although I would keep only_if_banned in unban API call just for improbable situation when bot will mess up some IDs and innocent user is removed from the group. Anyways let's continue discussion in the PR, if you think it is needed