Add a mypy-esque cache to Ratter

[BPHarris]

What

Ratter should cache it's IR for each Python source file to a JSON file, this cache should be invalidated if-and-when the source file's hash or the Ratter version changes, and when the file "passes" analysis.

This is distinct from the existing results cache created via --cache.

Cache Location

For a source file /path/to/dir/source.py, the cache should be stored in /path/to/dir/.ratter/cache/source.json.

Todo

This will require:

• Versioning info in Ratter
• Update the existing results cache (i.e. --cache, now --save-results) to also invalidate on version
• A decided, consistent structure to the cache (i.e. where the cache should be in relation to the source file)
• Prioritise reading the cache before processing again (prioritisation should be memory, cache, newly process)
• Tests
• Security notice on pickle/jsonpickle (+ info in --no-cache CLI arg)

Finally, the following changes must then be made:

• Update CLI #15
  • --cache should become --save-results
  • --no-cache to not use IR cache
  • --force-refresh-cache to forcibly refresh the cache
• Update CLI documentation