Arcade physics support slopemaps

I have added 3 types of slopes for now, the classic full square, HalfTriangleBottomLeft  and HalfTriangleBottomRight

Here is a code demo.

In your create method;
```javascript
	// map init
	var map = this.add.tilemap('main');
	map.addTilesetImage('Kenney 32x32', 'kenney32x32');

	// set the collisions, in my map 
	map.layers.forEach(function(l){
		var layer=map.createLayer(l.name);
	
		if(l.name==='collision'){
			var firstgid=map.tilesets[map.getTilesetIndex('collision')].firstgid;
			
			l.data.forEach(function(e){
				e.forEach(function(t){
					if (t.index < 0) {
						// none
					} else if (t.index - firstgid === 1) {
						// full square is by default
					} else if (t.index - firstgid === 15) {
						t.slope = 'HALF_TRIANGLE_BOTTOM_LEFT';
					} else if (t.index - firstgid === 17) {
						t.slope = 'HALF_TRIANGLE_BOTTOM_RIGHT';
					}
					// you could also add custom collide function;
					// t.slopeFunction = function (i, body, tile) { custom code }
				});
			});

			var collisionTiles = [];
			for(var i=firstgid;i<firstgid+18; i += 1){
				collisionTiles.push(i);
			}
			map.setCollision(collisionTiles, true, layer);

			this.tilesCollision=layer;
			//console.log(layer);
		}
		
		layer.resizeWorld();
	}, this);
```


The in your update method;
```javascript
	this.physics.arcade.collideSpriteVsTilemapLayer(this.player, this.tilesCollision);
```