Allow customization in Assets folder

[Pholith]

Hello, I would suggest to able the BuildDictionary() method to also check the Assets folder for new folder icons.
Because when we're using your package from PackageManager, it installs itself in /Package
The problem with /Package is that we edit only it locally, and new folders icons are not shared among others users of the project. (Plus it will be erased when updating the package)
This would fit with the readme instructions