Added `filesystem` and `gamedev` categories

- `filesystem` for my D:GameVFS library (could also add `virtual-filesystem` but I think that would be too specific)
- `gamedev` - various gamedev-oriented libraries may fit various categories, but I think it'd be good to have one category to be able to find gamedev libraries regardless of whether they are graphics, networking, physics, VFS as in my case or whatever.
Merge pull request #66 from kiith-sa/patch-1

Added `filesystem` and `gamedev` categories