Provide default compilers for various versions of Scala

[SrodriguezO]

Users can currently use Scala 2.12 quite easily by adding

bind(
    name = "default_scala",
    actual = "@rules_scala_annex//src/main/scala:zinc_2_12_8",
)

to their WORKSPACE file.

The rules would be more accessible if we provided similar default targets for other versions of Scala. Currently, users that need a different version of Scala must do all the configuration/set-up themselves.