Propagate server, handler initialization errors.
Allow config overrides via environment variables.

* Add `LoadEnvironment()` and an `Environment` type.
* Add the `env:` tag to all configuration struct fields. Environment
  variable names are case-insensitive.
* `Environment.Unmarshal()` uses reflection to decode configuration
  options specified via environment variables into a config struct.
* `Load{ConfigForSection, ExtensibleSection}()` now override TOML
  configuration fields with environment variables.

Overriding extensible section types is not currently supported.