Enable TinyDB class for subclassing

[ASMfreaK]

I want to be able to do something like this:

from tinydb import TinyDB
from tinydb.middlewares import CachingMiddleware
class YamlStorage(...):
    ...

class CachingYamlTDB(TinyDB):
    DEFAULT_STORAGE = CachingMiddleware(YamlStorage)

This PR enables TinyDB for such sublassing.

[msiemens]

That makes sense, thanks for the contribution!

[msiemens]

Now released in v3.5.0 🎉

[ASMfreaK]

Yay~
Thanks)