add optional global configuration #8

some thing that should be configurable:

• location of cache (usecase: each notebook/kernel should have it's own folder)
• hashing-algorithm (usecase: automatically add datum? use sha instead of plaintext)

How the configuration should work

• via global variables (maybe look at other magics first for best practices?)

import cache_magic
cache_magic.config["location"] = ".cache_magic_" + notebook_name
cache_magic.config["hash"] = hashlib.sha