Add ability to set nicknames for individual subscribers

[ff14wed]

This PR allows Subscribers to optionally specify a nickname.

For example, setting a nickname like follows:

Payload { OP: OP.Debug, CHANNEL: 9000, DATA: u8"TEST_CLIENT" }

will result in a response:

Payload { OP: OP.Debug, CHANNEL: 9000, CHANGED NICKNAME: TEST_CLIENT (subscriber 0) }

and log messages will contain the subscriber nickname like so:

[INFO] New subscriber connected: 0
[INFO] Filter set for TEST_CLIENT (subscriber 0): 0b11111111
[INFO] Disconnected: TEST_CLIENT (subscriber 0)
[INFO] Shutting down server because last subscriber disconnected

These are the restrictions on the values of nickname that are accepted:

• The bytes in DATA must be valid UTF-8.
• Must contain only ASCII alphanumeric characters or underscores (^[A-Za-z0-9_]+$)
• Must be 30 characters or less
• Names are allowed to be the same as other subscribers.