Getting an undefined config should throw an error

Nobody has claimed this yet.

Consider the following:

```js
const Config = require('@logdna/env-config')

const config = new Config([
Config.string('loglevel').default('info')
, Config.number('port').default(3000)
])

const foo = config.get('something-else') // happily returns undefined
// do something important with foo...
```

Rather than just silently failing in this case (e.g. a typo in the config name, or you just forgot to add it...) we should throw an error.

While there may be valid use cases for "dynamic" configuration in which the properties are _expected_ to be (potentially) unknown, the default behavior should be more strict.

Start by reading the Config constructor and the config.get entry point shown in the issue to understand how declared and unknown names are handled. Define the default error behavior while accounting for the mentioned dynamic-configuration use case, then add regression coverage showing that an undefined name throws and declared names still work.

Written by the indexing model from the issue text.