Port the Preferences API to Readium Web

[HadrienGardeur]

Now that the Preferences API has been well-defined and implemented in both toolkits in Readium Mobile, we need to port it to Readium Web as well.

This would strictly apply to the ts-toolkit and will need to be implemented both for the reflowable and the FXL navigators.

This is a key low-level API as it opens the door to good customization for the end user as well as future features (reader mode comes to mind).

[HadrienGardeur]

@JayPanoz this could be very useful for your current work on Readium CSS once implemented, as it would make it possible to build a playground on top of the Readium Web test app.