Add "preferred language" setting to limit the literals being returned

[agarciadom]

At the moment, accessing x.property where the property is associated to various RDF literals will return all the literals, regardless of the language tag. This can be confusing if someone is asking for the name of Spiderman and gets both the value in the default language and in Russian.

It'd be best to have a "preferred languages" setting where users can specify one or more language tags, and this could be set by default to [""], as in, just the default language. If someone wanted to first try getting literals in Russian and then in the default language if no Russian literals are found, they could specify ["ru", ""] instead.

[OwenR-York]

Within the launcher configuration, a user must be able to define an ordered list of preferred languages. This list of preferred languages will be used when processing RDF literals to return literals in the most preferable language.

For example, a list of {en-GB, en} is provided. Literals marked @en-GB would be returned before @en or @undefined

[agarciadom]

On second thought, it may be best to fall back to returning all values when no preferred languages are defined, and then have the new filtering behaviour if we have any preferences.

So if x.property does not have any RDF resources, it would work as:

• Have preferred language ["en"]: try to get the "en" values first, fallback to the untagged values.
• No preferred languages: get all the values without any filtering.

We may have to revisit this logic so the driver continues to work with non-string literals: this has been left to #19.