Add support for loader plugins

[maximumspatium]

Quoting the README:

See how to support other types of IDAPython plugins besides just processor modules.

Loader plugins would be highly appreciated. The problem is that Python loaders are hard to find. The existing loaders, covering widespread binary executable formats, are usually written in C++ so they need to be ported to IDAPython first.
Fortunately, stand-alone Python libraries for parsing several common executable formats exist:

• pefile for parsing M$ PE files
• macholib for Mach-O executables

Creating basic loader plugins ontop of them should be easy. The existing pyelftools based ELF loader serves as a good example of how to do that.
The first step is to enable support for loader plugins in ScratchABit...

[maximumspatium]

Well, it looks like the structure of the ELF loader in ScratchABit already resembles the IDA loader API. It's difficult to say whether a true IDA API is required or not because I'm currently not aware of any real-world IDAPython loader.