Add option to specify bid duration

[AfkF24]

Basically what the title suggests. Having an additional parameter for players to set the maximum duration for their items would be useful.

/ah bid <price> [amount] [duration].

• If [amount] isn't specified, [duration] would be used (and vice versa)
• The duration should be lower than Settings.Bid-Time (default bid time, if not specified)
• For ease of use, the duration format should be as simple as 10m, eg. /ah bid <price> 10m or /ah bid <price> 5h