Added --require option to the Cakefile.

I have made [a compiler hook adding Scala-like placeholder syntax](https://github.com/taku0/placeholder_syntax_for_coffeescript).

The hook should have no impact for the existing code, so that I added the `--require` option to the Cakefile to see the hook passes cake build:full.

Adding experimental language features as a plugin would be lightweight than modifing the compiler.  To encourage language experiments, would you merge the commit?