Environment Variable config

[moskyb]

This PR: Allows users to specify their buildkite API tokens and organization slugs via environment variable, allowing them to end-run around the bk configure command. This should make running the bk cli in headless environments (like CI jobs 👀) much easier.

Environment variables take precedence over config items specified in files, as is fairly standard across CLI tools.

Example:

$ rm ~/.config/bk.yaml
$ export BUILDKITE_API_TOKEN=bkua_haha_nice_try 
$ export BUILDKITE_ORGANIZATION_SLUG=bennos-extra-secret-org 
$ bk pipeline view "my-cool-pipeline"
  My Cool Pipeline: Possibly the coolest pipeline around
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

                         Speed: 11s    Reliability: 80%

    agents:
      queue: default

    steps:
      - command: buildkite-agent pipeline upload

Additionally, it adds another envar for BUILDKITE_REST_API_HOST, similar to the graphql host envar added in #406. This should make local development easier.

🍷🧀 This PR should pair with #407 nicely - they allow gradations in how interactive you want CLI configuration to be

[mcncl]

Was there a reason for changing this? I think we only ever give it strings, but in its current form it could be used with any comparable type.

[moskyb]

nothing other than YAGNI - i figured if we weren't using generics we probably shouldn't use generics, but i'm more than happy to change it back if you like

[mcncl]

Nah, not at all, think it was in as a just in case, but we can re-visit if it's ever needed