Add a 'simple' and 'disabled' stateful configuration mode

[caronc]

💡 The Idea
A global environment variable such as APPRISE_STATEFUL_MODE that defaults to hash (to continue storing as it does today) but with the additional value of:

• simple which writes the files straight to disk using the same keyname they were accessed by (and uncompressed) making it easier for people to also open the file on their system and edit it that way too.
• disabled which disables the ability to write stateful configuration in it's entirety.

🔨 Breaking Feature
n/a