Add user blocking feature

[iulianalexa]

This pull request addresses Issue #369 and implements the requested feature.

To block a user, add this line to the config:

blocked_users: ["~user@example"]

Inside the blocked_users list, add the hostmasks of the users you would like to block.

If this gets approved, we could eventually expand on the block function by implementing features such as:

• Regex matching (e.g. "~user@*")
• Separate .yml file for the blocked users
• /block, /unblock commands