Add sqlite feature flag and document own store implementation

[mFragaBA]

I think currently rusqlite comes in both the CLI and the library. We use it to run integration tests for example. It's sort of our "default" store implementation. That's why I find it reasonable to put it under a sqlite feature flag. Perhaps it's better to make an issue to address this in the documentation as well as putting the sqlite store under a feature flag. What do you think?

Originally posted by @mFragaBA in #262 (comment)

[bobbinth]

As a part of this, we may want to break out dependencies which are needed for the CLI binary and then turn on relevant features for a specific use case.

[bobbinth]

This should be closed once #367 is merged.

[igamigo]

Closed by #367