Make Spotify segment more error-proof
Rework memoize class

- Removed *args: segments are not receiveing any non-keyword arguments so no
  need to bother with *args at all.
- Replaced additional_cache_key with cache_key: all functions that formerly used
  additional_cache_key appear not to need initial arguments as cache keys. It
  also simplifies the code.
- Made _cache be defined in __init__, it is much easier to implement cache
  purging this way.

Ref #205
Fixes #208
Remove 'before' key from file_vcs_status segment

Fixes #219