View Other User's Inventory

[LeoSaucedo]

Implement ;inv <user> with the following spec:

• If the parameter mentioned is a valid user, display their inventory.
• If the parameter is not a valid user, throw an error.
• Must use existing getInventory functions.

[bryandean80]

added here https://github.com/LeoSaucedo/rikka-bot/pull/83/commits/9277c738845ebad12ac1b0696dc27cec31067ab7