Add `init` and `new` commands

It would be cool to have an `init` command for `dotpkg`, roughly analogous to `npm init` that generates a template `dotpkg.json` for the user in the current directory.

Similarly, a `new` command that takes a name for a new `dotpkg` in a corresponding subdirectory like `stack new`, would be convenient.

`dotpkg new my-package` should be equivalent to `(mkdir my-package && cd my-package && dotpkg init)`.