Distinguish default properties and custom properties

[Python-ZZY]

I believe it's necessary to add a dedicated method for retrieving an object's custom properties. When I attempt to access the properties, I get not only its custom properties but also the tile's inherent attributes (such as id, source, trans, width, etc.), all mixed together in the properties dictionary. After all, the custom properties supported by an object usually vary, and developers often define these properties in the Tiled map file before directly passing them to the corresponding Python class via ​**​object.properties. Attributes like id and source are often unnecessary in this context.

In addition, I really hope pytmx can support templates. This issue has been brought up by others.

[joereynolds]

If this is what I think it is, I opened up a PR for that here
#188

I've been working off of a fork for about a year until this gets merged