Improvements

[KanTheAstronaut]

• Changed admin id from int to long (also in the example) since telegram user ids are not integers
• Instead of returning null when we can't find a user in RemoveBotAdmin and AddBotAdmin let's return "Unknown user or user is not cached!" (because we are checking the database for the target user) or "You can't add/remove yourself!"
• Changed "GetTarget" so it returns null instead of returning the source user (which made the null check pointless)
• Made file logging optional (added an option in DefaultBotOptions and the interface)
• Made loading default modules optional
• Made loading addon modules optional
• Fixed a few typos

Disclaimer: I did not test all of the above changes (only the 1st, 2nd, and 3rd) but none of them should cause any exceptions