Add nodejs as a dependency of ruby-execjs. Close #8.

Although upstream says nodejs is one of the optional dependencies
everyone seems uses execjs with nodejs. Let's make it hard dependency.
Add optdepends support

This allows to add optional optdepends to package config. Closes #4.

Example of usage:

$ cat config.pkg/mypackage.yaml
optdepends:
  valgrind: Valgrind support
  ruby-sinatra: Sinatra web framework support
Quarry repo cleanup is implemented some time ago
fog: Add unf as optdepends

See https://github.com/fog/fog/pull/2320
Merge pull request #9 from daurnimator/master

fog: Add unf as optdepends