Build persistance backend

[lhaze]

• [ ] SQLite as a storage
• [ ] data source using db storage as a cache
• [ ] config.Config supports building data source with a db backend

[ffigiel]

Implementing this is going to be tricky:

• Editing comments. Perhaps we should allow negative time log entries, e.g. :clock1: -2h30m | Whoops, I logged too much time
• Merge commits with old branches unknown to our storage.

We can get around this by occassionally refreshing all the data, but for bigger projects this would take forever.

[lhaze]

@ppeczek [OT] There's a slight difference between persistence backend and persistent backend. We don't want our backend to be persistent, whatever that would mean. We want backend to provide persistence for our library.

@megapctr I'll address separate issues to cache invalidation cases, legality of proposing entries (probably some kind of authorization rules in configuration) and correct entry analysis in the event of merge pulls or reversions. Each of these is a serious problem to usefulness of our project. For now, let's stick to providing any persistence.