Add support for external linking to anchors in Javadoc and Scaladoc

[pikinier20]

Here are some examples how linking to members in Javadoc and Scaladoc looks like.

https://docs.oracle.com/javase/8/docs/api/java/util/List.html#get-int-
https://docs.oracle.com/javase/8/docs/api/java/util/AbstractCollection.html#addAll-java.util.Collection-
https://docs.oracle.com/javase/8/docs/api/java/util/AbstractCollection.html#containsAll-java.util.Collection-
https://www.scala-lang.org/api/current/scala/collection/immutable/List.html#:++[B%3E:A](suffix:scala.collection.IterableOnce[B]):CC[B]
https://www.scala-lang.org/api/current/scala/collection/immutable/List.html#appended[B%3E:A](elem:B):CC[B]
https://www.scala-lang.org/api/current/scala/collection/immutable/List.html#appendedAll[B%3E:A](suffix:scala.collection.IterableOnce[B]):List[B]