[WIP] Steam API integration

[rilifon]

This PR adds an integration for our game with Steam API. It uses LuaJIT and steam's DLLs. For it to work, you first need to add these two DLLs:

• steam_api.dll and
• steam_api64.dll into your love .exe folder. After that, you can run the game with passing the argument --steamdev and it should connect with Steam's API and the Steam overlay should appear. Note: You must have Steam launched and active in your computer for this to work.

Also added the --steam flag to run the game natively on Steam. This will only work directly on Steam since this won't create the auxiliary file detailing our games ID (for some reason Steam says we shouldn't create this file on the uploaded build, maybe to hide our game's AppID?)

When completed, will close #238

[rilifon]

As pointed by the dev team, this should only work on Windows... So I'll guess i'll try to make it work on other OSs before merging with the dev and closing the issue