Add mods support.

Well, technically we have no mods so it does nothing unless you're using local. Also, this can only override monsters (post-unification)

So it does nothing right now but add a compatibility layer so e.g. HoraK and Livio changes can bypass the unification. ManaPlus does support declaring the same monster twice, the last declaration prevail, and mods are loaded last.

At same time, if something absolutely must not be overriden by a mod (like a mimic ─ not that it makes any difference under Evol2, but it does under TMWA), developers can just define it after the include and BAM! Mitigated.