Use appropriate version of the Scala.js Linker

[dubinsky]

Auto-configure scalajs linker the way Scala plugin does for zinc:

• use the one configured by the project on the scalajs configuration or supply default;
• deduce the default based on the version of the Scala library configured by the project.

See for reference:

• https://github.com/gradle/gradle/blob/master/subprojects/scala/src/main/java/org/gradle/api/plugins/scala/ScalaBasePlugin.java
• https://github.com/gradle/gradle/blob/master/subprojects/scala/src/main/java/org/gradle/api/plugins/scala/ScalaPlugin.java
• https://github.com/gradle/gradle/blob/master/subprojects/scala/src/main/java/org/gradle/api/tasks/ScalaRuntime.java

[dubinsky]

starting with 0.0.4, both zinc and ScalaJS are used dymamically