Fix #426: Cross-compile the compiler with full Scala version.

Because the Scala compiler API is not binary compatible between
minor versions, compiler plugins should be cross-compiled
against the full Scala version (not the binary Scala version).
Add "warn" method to test reporting interface
Add option support to test framework.

Using testOnly in sbt, you can pass arguments to the test
framework. This commit levarages this by:

- Adding an option to disable sourcemaps (fixes #431)
- Passing additional arguments to the JS test framework
Merge pull request #444 from sjrd/full-cross-compile-compiler

Fix #426: Cross-compile the compiler with full Scala version.
Merge pull request #445 from gzm0/test-options

Test options