Move CLI stuff into its own dir

I think having it in its own dir like this means that people can include it in their own projects as they wish. Even though the CLI does nothing currently.
I think this should just be `main` and the file should just be `lifx.go` (to create `lifx` binary). The indirection in the root of project doesn't buy much. Then `go install github.com/bjeanes/go-lifx/cli` should just work, I think.