wrapped metalsmith CLI in node-liftoff

I wrapped the metalsmith CLI in [node-liftoff](https://github.com/tkellen/node-liftoff). This allows the use of a global metalsmith installation with local `metalsmith.json`s without the need to install all the plugin globally, which is more convenient than the old  `node_module/.bin/metalsmith`. It basically works like gulp.js now, where we load a local version of metalsmith when calling `metalsmith` from the command line.