Feature/blacklist

[amekusa]

Hi. I love this plugin! I have a request for a feature that prevents users selling specific types of items. In other words, a blacklist. So I implemented it.

This pull request suppose to:

• Enable the server owner to specify multiple item types that he/she doesn't want players to sell
• Prevent players adding blacklisted items to the market via the command: /um add

If you like this idea, please merge this request. Thank you.

[amekusa]

Thank you!