GH-154 Allow override of the descriptor validation link extractor

[romeara]

Address the request in #154 by allowing override of the link extractor used with descriptor validation, defaulting to the current behavior of using HypermediaDocumentation.halLinks(). Override is done via a field st]et in ResourceSnippetParameters, with associated builder methods, and consumed in DescriptorValidator in place of the previously hard-coded halLinks() value.

I am unsure what tests would be appropriate to add alongside this addition - however, all existing tests pass.

Please note, this is my first attempt at Kotlin in general, and I do not have Kotlin tooling set up - I did these edits by running Gradle wrapper from the command line and with an Atom text editor. I 100% expect I did some stuff wrong and adjustments will be needed, and I am hoping it conforms to whatever formatting standards are expected (I didn't see any contributor documentation, so just tried to mimic the existing code as best I could - if such documents exist and I just missed them, please feel free to point me at them so I can try to address anything in there this PR doesn't conform to).

[coveralls]

Coverage decreased (-0.04%) to 92.593% when pulling 7bb5cf9 on romeara:feature/romeara/154-overridable-link-extractor into af19c25 on ePages-de:master.

[ozscheyge]

Hey @romeara ,

wow, that was fast (esp. for no prior Kotlin experience+setup!)

I'll have a more thorough look tomorrow, maybe I can come up with a test case.