Improve YAML storage syntax (and make the BibtexParser compatible)

Currently, the YAML format used for storage is not being leveraged to its full extent. I noticed this especially during the development of !19 (merged) where I would really liked to have specified multiple files as a YAML list, rather than a comma-separated string.

However, at that time this would have led to an over-complication in the parsing from and to bibtex files because the bibtexparser module cannot handle lists as its input.

Thus, when implementing #49 (closed) we should consider extending towards these possible use cases of the YAML format. I think it would ease readability (because I myself tend to look at the plain YAML format quite often, actually) and maintainability from a user perspective (since the edit command will provide the plain YAML).

We may also want to link this to #54 (closed) as the improved configuration options will likely simplify handling different format variants without breaking existing user databases.