Changes

Add dTrade object and related tags, mechanisms, and command (#1892)
* Add dTrade object and all dTrade-relevant tags/mechanisms

* Add opentrades command

* Remove pointless copy-pasted line

* Prevent versions below 1.12.2 from accessing dTrade features

* Remove pointless version check code

* Remove almost-forgotten unneeded imports

* Meta fixes and EOF newlines

* Fix code and update to latest Denizen core commits

* Remove unneeded lines

* Just don't allow the selected_trade tag to be used at all
 The inventory casted to a MerchantInventory doesn't return a null, but
apparently the MerchantInventory#getSelectedRecipe() method likes to die
so hard that Denizen consistently throws a NullPointerException error
stacktrace. So I'm just not going to let people access that, ever.

* Don't let inputs default to two air items

* Let's not be stupid when seeing if an entity is a Merchant
 (commit: e42972a)