Bot interaction: set item priority thresholds

Allow a player to set thresholds (per-world) for a received item that adjusts the priority based on the total count of that item that has been received.

e.g., "ammo" for Super Metroid:

• A certain minimum number of Super Missiles, Power Bombs may be marked as "high"
• Pickups beyond threshold should be downgraded to "medium", and beyond another threshold to "low"

Respond with:

• Error message if the the player's world context is not set, any priority or count is invalid
• Success message (or affirmative reaction) after setting the new item threshold(s)

Possible command syntax: !item-thresholds "item name" count:priority count:priority ...