Generate <classname>.cfg/userconfig.cfg files at runtime if they don't exist

 * Closes #87 in conjunction with fortressforever/fortressforever-scripts#3
"MOD" is a weird constant for telling the filesystem which search paths to check (options seem to be "MOD" and "GAME"). I can't really find much documentation about it, but I did find this in dlls/navfile.cpp:
```
	bool navIsInBsp = false;
	FileHandle_t file = filesystem->Open( filename, "rb", "MOD" );	// this ignores .nav files embedded in the .bsp ...
	if ( !file )
	{
		navIsInBsp = true;
		file = filesystem->Open( filename, "rb", "GAME" );	// ... and this looks for one if it's the only one around.
	}
```
We pretty much always use "MOD" in FF files.