This commit adds a new --mac_root_key flag to both the lncli create and
lncli createwatchonly commands that allows the user to specify the
macaroon root key that should be used when creating the macaroon
database on wallet initialization.
This allows for deterministic wallet initialization and baking of
macaroons before the wallet is initialized.
We want to export some of our CLI code to re-use in other projects. But
in Golang you cannot import code from a `main` package.
So we need to move the actual code into its own package and only have
the `func main()` in the `main` package.
