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.
`decodeEnvField()`: Support extensible sections.

* Environment variable names are now constructed from section names.
* Explicit `env:` tags are no longer required. The field name will be
  used if the tag is omitted.
Add `splitList()`.

Ensure `GomemcStore` handles multiple servers correctly.
Extract environment variable logic into `envconf`.
Merge pull request #84 from mozilla-services/feature/config-env

Allow config overrides via environment variables