Migrate to null-safety

[JonasWanke]

I tried to keep the methods as close to the pre-null-safe-versions as possible, though maybe some of it should be changed with regards to #3.

This migration includes a breaking change: As in the null-safe version of shared_preferences, setters no longer accept null to mean removing values. (remove(key) can be used instead.)

[roughike]

Hey! Thanks for the PR, good stuff. My time is limited, but I'll try to review when I can.

For those who want to start using this today:

pubspec.yaml

dependencies:
  # ...
  streaming_shared_preferences:
    git:
      url: https://github.com/roughike/streaming_shared_preferences
      ref: a460743

[josh-burton]

This is becoming a real blocker in a lot of projects. Would be great to see this merged

[roughike]