Add option to create project from package

This internally invokes Composer's `create-project` command which allows for
creating projects based on other packages. To reduce the number of options the
shorthand syntax `<package>:<version>` supported by Composer is promoted.

Fixes #8
Actually `create-project` already takes care of running `install` but we need to make sure that on subsequent runs `install`/`update` is run. And this will actually fail right now if you `$ensure` `latest`. Gotta fix this.