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
Oh yes, but we also need to check weaker fields for same "(move_cost( x + a, y + b ) > 0 || has_flag("PERMEABLE", x + a, y + b) )" , if here is no way for field to be on wall tile, but somehow there is, it shouldn't be "fed" by nearby one.