Add option to disable TypeScript

When transpiling a .js only codebase with AwesomeTypescriptLoader, the end result is incorrect. I did not dive into the exact reason since the overhead added by using ATL is such that it is easier too simply make it optional and fallback to Babel (which is already used by ATL for .js files).
This also simplifies the sourcemap workaround (a second remap step) currently used in karma for a hybrid (.ts and .js) project.

The added bonus here is an improvement in build speed.