Per-collection and per-profile configuration

[LucasPickering]

Discussed in #723

Some configuration fields should be overridable by collection and profile. Here's an example:

config:
  persist: false # Maybe this is a sensitive collection and you want nothing persist

profiles:
  dev:
    name: Dev
  staging:
    name: Staging
    config:
      theme:
        background_color: yellow
  prod:
    name: Prod
    config:
      theme:
        background_color: red

This would create a configuration hierarchy (lowest -> highest precedence):

• Default values
• Global config.yml
• Collection config
• Profile config

This would be useful for:

• Collection- and profile-specific themes
• Overriding content type for a particular collection

However, there are some config fields that absolute should NOT be overridden by collections, for security reasons:

• ignore_certificate_hosts
• commands.*
• pager
• editor

There are some fields that probably will not be useful to override by collection (e.g. input_bindings), but as long as it's not dangerous I'm inclined to allow it.