Add `init` and `new` commands

[fwcd]

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).