Add ability to configure UserSettings default values

[ettore]

This allows apps to be able to set their own initial values for every UserProperty.

[ettore]

This should be a non-breaking change because the existing values are preserved, and the api signatures are compatible with the previous ones via default param values.

The documentation mentions "removing objects from userProperties", which is something made possible by the related r2-shared PR: readium/r2-shared-swift#157.

The documentation for pageMargins and lineHeight lacks the unit of measure but I wasn't sure which one (px, pt, em, etc) ReadiumCSS is using.

[mickael-menu]

Looking good, I just moved the userSettings param to the EPUBNavigatorViewController.Configuration object, instead of the navigator initializer.