Added waiting message to various commands that perform I/O

[riddla]

Refs #807, #1167

• moved waiting message from list command to util class
• now alias ls
• now alias rm
• In the Now util class:
  • listAliases
  • listSecrets
  • list
  • listInstances
  • logs
  • listDomains
  • getDomain
  • [...]

[riddla]

While working on the util/index.js I wondered if the various waiting (and error?) messages should be encapsulated somewhere. Lookup table within the same or another file? Or a more general purposed module that holds the various message strings and makes them easy to edit and maybe align in one place?

[rauchg]

@riddla we indeed went with that approach. Please take a look at certs.js or alias.js for an example of how we want to do logging and error-handling moving forward.

We want to deprecate util/index.js altogether, but we are doing so incrementally.

I'm closing this PR as the codebase has diverged quite a bit. But feel free to submit a similar PR against, for example, now dns, which we haven't gotten around to refactoring quite yet.

Much appreciated 🙇