Add property name translation for PLYLoader

The property names in PLY files are given as strings, some
implementations may use different names for common properties.
This commit adds a translation mechanism based on a map,
which is specified via the constructor.

Example with no translation:

```javascript
  loader = new THREE.PLYLoader();
```

Example with translation:

```javascript
  loader = new THREE.PLYLoader({
	  		diffuse_red: 'red',
			diffuse_green: 'green',
			diffuse_blue: 'blue'
		});
```