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
Line 372 says "if (current_density > 1 && current_age > 0 && !spread.empty()) {"

The "current_density > 1" is what is preventing gases from meandering. It seems like it's been like that since the function was written. Perhaps the "spread" code was never intended to allow gases to meander. If that's the case, then this PR is done!