Add per project CLI configuration

[cbetta]

This config could hold:

• Preferred URL host
• App key

Related issues:

• CLI enable environment selection #114
• [Discuss] Create Nexmo Application config file upon app creation #109
• Allow user to configure default size param #51

Gut feeling says we can put this in the local .nexmorc we are already looking for

[cbetta]

Changes to be made:

• Update config loading to check BOTH local and global .nexmorc. This way we could have API keys in ~/.nexmorc and host settings in the local ./.nexmorc.
• Add a way to write new config to local or global files.
  • Set: nexmo config:set key=value
  • Unset: nexmo config:unset key
  • List: nexmo config
  • Get: nexmo config:get key
  • Global: nexmo config:set key=value -g
• Use config to set/apply environment selection (see CLI enable environment selection #114)
• Use config to set/apply default pagination size (see Allow user to configure default size param #51)
• Use this config to potentially store details about keys (tbd, see [Discuss] Create Nexmo Application config file upon app creation #109)

[cbetta]

Remaining questions:

• Currently nexmo setup writes to a global file, and nexmo setup --local to a local file. This would be opposite to nexmo config:set as proposed.
• What should be done if there's no global file but there is a local file and someone sets new config?

[leggetter]

API key and secret are account level credentials where as application ID and public/private keys are Nexmo application level config entities. Page size is a CLI specific thing. This proposed solution is suggesting adding a way of overwriting account/global config at an application level and also allows some CLI config to be set in the same place 🤔

I'm a little concerned about merging account, CLI config, environment/API host config and application config. Should they be very clearly separated?

My feeling is that we keep #109 focused on application config and separate from this issue. I do think this issue can address account, CLI and environment config.

What should be done if there's no global file but there is a local file and someone sets new config?

Consistency suggests that config:set should default to global unless a --local flag is passed. Maybe the CLI could warn if config:set is called and there is local config?