Allow for local, per-project plugins

[zimLeTux]

Edit: I am currently reworking this as I have realized this relies on a dist-info present in the working directory, which is not the wanted behaviour at all !

This small change allow to have custom converters in a given project.
It is more like a proof of concept and should probably be reworked/cleaned up.
Project plugins should be located in a "plugins" directory.

Current limitations:

• the plugin class name must be the same name as the file, but starting with a capital letter ;
• plugins can only be located in the plugins subdirectory, and nested subdirs are not yet possible ;

Some use-cases:

• Have a tool to create NPC (both mesh and in-game properties, or even scripts) and export them automatically ;
• Mesh generation based on parametric files ;
• Conversion of table-based data (for instance an ODS table listing all the items or monsters) to a more game-friendly format (XML, json, SQLite...)
• ...

[zimLeTux]

Rework is complete!

The limitation on the filename/classname is removed: as long as the file is in the plugins directory and contains a class whose name ends with "Plugin", it is available.
Maybe the name of the plugins directory should be configured in the .pman file?

[Moguri]

Why not have the project register entrypoints for its custom plugins in its setup.py (or equivalent)? This should work with something like:
pip install -e .

[zimLeTux]

It is actually (almost) what I did in the first version, but it requires the user to re-run pip each time they add a new plugin (which might not be a real problem).
I can reimplement this behaviour, but as a user I think it is better to just set a directory where plugins are located and have your plugins "just work".