Add persistent storage

[reibitto]

Some commands could benefit from persistent storage. For example, it would be nice if you can add new snippets from Command Center itself like snippet add "keyword" "value" and so on.

I'm leaning towards SQLite. Don't want to reinvent the wheel here.

Maybe we hold off on this for a little while until Command Center gets closer to being feature complete. Because things are still changing at a fast pace and I don't really want to deal with breaking changes and SQL migrations at this stage of development.