fix unknown environments polluting config keys

envcfg will mix in the '*' params in unknown environments but will also add keys to the config object for all other environments, so, in the current version, this:

```
var envcfg = require('./');
var cfg = envcfg({
	'*': {
		'my-key-here': 'is-important'
	},
	'someEnv': {
		'dude-bro': 'what is up'
	}
});

console.log(Object.keys(cfg));
console.log(cfg['my-key-here'])
```

Would print:

```
[ '*', 'someEnv', 'my-key-here' ]
is-important
```

Rather than just...
```
[ 'my-key-here' ]
is-important
```

...which is helpful if you, like me, need all available config params.