Suggested addition to FlxTilemap - getTileCollisions - closes #559

I started an issue on this over at https://github.com/HaxeFlixel/flixel/issues/559, so I submit this for consideration. Right now there is no way of accessing the allowCollisions value for tiles outside the class. This is something I use extensively, for recognizing ledges the player can grab on to, creating drop-down floors and calculating simplified collision detection for NPC characters, and I just find it silly that this data is tucked away under a private member.

So in order to save other people the head-ache, please consider adding this little function that gives read-only access to tile collisions.