Refactor the config file to make it more intuitive and structured

[soywod]

The first big problem is the untagged enum used for backends (#295 (comment)). This leads to unprecise and hard-to-debug errors like #333. By using an adjacently-tagged enum we avoid the parser to "guess" the right config variant and get better error messages, but we break the config API.

The second problem is that options have been added on the fly since the beginning, and most of them are misplaced. We need to make the API more consistent by grouping options by context. Also some namings are not the best. default_page_size is a good (bad?) example: default_ is not need since it is implied by the config itself, and we miss the context (listing).

[soywod]

Released in the v0.6.0 🎉 please read the changelog because this feature brings breaking changes!