Allow emitting .js extension in module paths in compiled output

[b-strauss]

Module loaders like systemjs expect module paths to point to a valid file path, including the .js extension. Although exploringjs says the extension might not be needed, the module loader spec's default behaviour has not yet been decided upon. So for code to have the widest possible support it would be good to have an option to tell the compiler to add the extension.

[vladima]

looks like a duplicate of #4595 (this comment)