Support external analyzers

[berggren]

This issue is tracking the design and work on supporting external analyzers. There are two options for this:

1. analyzer code hosted elsewhere than in the official Timesketch repository. Today we host these unsupported analyzers in a contrib directory.
2. Decouple the analyzer framework from the main codebase.

[jaegeral]

Will move that conversation / Idea to "Future" for now.