Allow gases to travel through fences, wire gates, bars [WIP]

As per the title says. I did not test this... but what can go wrong??? I changed so little! :D 

I've added a PERMEABLE tag to terrain that allows gases to flow through. Danzer suggests allowing blobs to travel through these; maybe this will be a future use for the permeable tag.

This was brought up my IRC members MShock777 and Atzerkins!

To do : 
- Properly add the flags, so this actually compiles
- Document the flags properly
- Disallow fields spreading to other fields if move_cost is not greater than 0
- Consider allowing gases to leak under doors