Optional cache + refactor.

[ryan-roemer]

• Make better-sqlite3 and optionalDependency. Switch to noop cache if not present. Fixes #49
• Add Cache.wrapAction helper for common use case of "try cache get, do action, set cache".
• Change cross-process communication to just serialize/deserialize the applicable cache instance.
• Various refactoring and cleanup.
• Re-enabled eslint rules for better hamonized style.
• Add cache option for InspectpackDaemon.create.
• Add error logging for worker errors.

/cc @tptee @gartz

[ryan-roemer]

Released in inspectpack@2.2.0