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
i just realized we have no reference documentation in the README.
i wonder if it's time to start using puppet-strings. (addendum: after trying puppet strings: "what is this?? how do i use it?!?! it's not like documented!!!")