Make a real texture pack system

[Unarelith]

Currently, the only way to use custom textures is to replace those in mods/default/textures.

However, this only works for blocks/items textures.

A real texture pack system should be able to:

• Support different textures sizes (eg. mixing 16x16 and 32x32)
• Change GUI textures
• Change font

Once it's done, replace every textures by Faithful 32x ones, and provide a way to use original Minecraft textures from a legit copy of the game.

[Unarelith]

The last commit added the possiblity to load a texture pack with -t/--texture-pack.

The next step will be to do all the things mentioned above.