Use local database for caching

[mscottford]

I'd like to use a local database for caching data that we query from remote sources. This will improve performance while also reducing the chances that we are overloading those remote sources for data that hasn't changed.

I'd like to use LiteDB for this effort: https://github.com/mbdavid/litedb. And I think it would be great to store the database at ~/.freshli/cache.db by default but give users the ability to override that.

We could also change the application/library to use that datastore for querying information. There's a chance that doing so would be faster than the way we're doing it now, at least if it supports indexes.

I think that the following information would be cached here:

• package repository data (package versions and release dates)
• git repository data (manifest contents organized by date changed)

[mscottford]

And when we start grabbing vulnerability data, that would go in there, too.