Add basic customization support, integrate with names

This MR lays the groundwork for class/function/enum customization. Another key is added, "customization", which will have other customization related fields as values. Currently, the only customization key is "name", which customizes the name on the Python side. This was already a feature, but it has been moved for the sake of consistency.

The "cust_name" field was also renamed to "name", since it is a key of the "customization" dictionary. The default custom name for a function/class/enum has been switched from {cpp_name}_py to just {cpp_name}

Related to #129 (closed), !80 (merged), !76 (closed)