Configurable eBook data file

[jdormit]

As mentioned in #19, there should be a file that stores per-book data, such as annotations. This is in addition to the data.json file, which stores global application data and lives in the user data directory. The new data file should have a default location, but also be configurable from a preferences file (~/.livre). The idea is that this data could be synchronized between machines through DropBox or something similar.