Add case insensitive matching for env vars

Hey !

It would be just great to add the capability to load env vars with no regards for the case of those.

As of today, the following tepee is able to load its value from the `$FOOBAR` env var:

```
class MyApp::Configuration < Tepee
  add(:foobar, 42)
end
```

It would be awesome if it was as well capable of loading `$foobar` or `$FooBar`.
