Add a config that overwrites used slot for diging

Currently baritone picks whatever tool is fastest for the job.
* but in some cases you might want to use a less expencive pickaxe or one with special properties like silktouch.

* It would assure support with anything custom with command blocks/datapacks/plugins/mods.

* This would get around a anti-cheat implementation where servers switch the function of items and cover it up with a resource pack.

(also would likely fix a crash with ic2 #646 )

No file, test, or entry point is named in the issue. Start by clarifying the intended configuration and how it should interact with tool selection, custom items, and the crash reported in #646; done should include the requested override behavior and coverage for the relevant cases.

Written by the indexing model from the issue text.