Username obfuscation in 'leaderboard' output

[jbirdkerr]

Give users the ability to obfuscate the output of the 'leaderboard' command to prevent the Slack "highlight & ping" mechanism from alerting users every time someone checks the leaderboard. Obfuscation in this case could be "jbirdkerr" instead of "jbirdkerr".

[dblock]

Users can now set their own nicknames via set nickname [name], so those annoyed by the notifications can just rename themselves in leaderboards. Supports emoji.