[4.2][!gear] allow weapon aliases as parameter of the !gear command

To ease the !gear command usability, weapon/item keys alias can be defined.
For instance, one could type `!gear +de` while someone else could type `!gear +desert` to allow the _Desert Eagle_.

Also:
  - add tests for the 4.2 specific !gear implementation
  - Update doc by adding a 4.2 specific doc section for the !gear command