Add support for properties of a Link Object

[Hadrien Gardeur]

Currently, the properties of either a spine or a manifest item are ignored during the parsing.

Instead of this behavior we'd like to:

• turn a number of these properties into rel values, for instance for the cover or the ToC
• express the rest of them in a more consistent way, for example properties such as mathml or svg could instead be expressed as "contains": ["mathml", "svg"]

Additional work on the Web Publication Manifest will have to be done in parallel in order to improve how properties are expressed in our publication model.